Ultra-high Strength Steels
Ultra-high Strength Steels
Structural steels with very high strength levels are normally called ultra-high strength steels (UHSS). The designation ultra-high strength is arbitrary since no universally accepted strength level for the term has been established. Also, as structural steels with greater and greater strength have been developed, the strength range for which the term is applied has gradually increased.
UHSS is widely used in automotive industry, engineering machinery, mine exploitation, military, and aerospace industry because of its good performance, such as high strength, strong toughness and good ductility. It is reported that the automotive industry presently uses UHSS with yield strengths of upto 1,400 MPa. The ultra-high-strength structural steels are quite broad and include several distinctly different families of steels. In this article only medium carbon (C) low alloy steels, medium alloy air-hardening steels, and high fracture toughness steels are described.
The use of UHSS in automotive manufacturing has the greatest potential to reduce the production costs. The development of UHSS with higher strengths and ductility makes it possible to manufacture single and lighter components where traditionally, components are joined together. In addition to reduced welding costs, this has the added advantage of reducing regions of high stress concentrations brought about by hydrogen (H2) embrittlement introduced through welding. Crash performances are boosted by the additional strength while weight reduction is achieved through thickness reduction at the same time. The large orange area in Fig 1 shows the region where UHSS normally fit in within the elongation-strength diagram. The aim is to improve the strength range without compromising formability while simultaneously reducing the overall production costs and improving weldability.
Fig 1 Schematic diagram of total elongation and strength for different steels
The effects of thermo-mechanical treatments such as ausforming and hot and cold working on the properties of several UHSSs have been studied at length. With sufficient deformation while the steel is in a metastable condition, strength levels not attainable by standard quench and temper treatments have been obtained, very frequently with higher ductility and fracture toughness than those normally expected at these very high strength levels. However, such thermo-mechanical treatments are not widely used commercially, presumably because of practical difficulties in adapting experimental techniques to actual production.
Medium carbon low alloy steels
The medium C low alloy family of UHSS includes grade 4130, the higher strength grade 4140, and the deeper hardening, higher strength grade 4340. Several modifications of the basic 4340 grade steel have been developed. In one modification, silicon (Si) content is increased to prevent embrittlement when the steel is tempered at the low temperatures required for very high strength. In certain steel grades, vanadium (V) is added as a grain refiner to increase toughness, and the C is slightly reduced to promote weldability. One steel grade contains V, slightly higher C, chromium (Cr), and molybdenum (Mo) than 4340, and slightly lower nickel (Ni). Other less widely used steels which can be included in this family are grade 6150 and 8640 steels.
Medium C low alloy UHSSs are readily hot forged, normally at temperatures ranging from 1,065 deg C to 1,230 deg C. For avoiding stress cracks resulting from air hardening, the forged parts are to be slowly cooled in a furnace or embedded in lime, ashes, or other insulating material. Prior to machining, the normal practice is to normalize at 870 deg C to 925 deg C and temper at 650 deg C to 675 deg C, or to anneal by furnace cooling from 815 deg C to 845 deg C to around 540 deg C if the steel is a deep air-hardening grade. These treatments impart reasonably hard structures consisting of medium-to-fine pearlite. In this condition, the steel has a machinability rating of around 50 % that of the free cutting steel.
A very soft structure consisting of spheroidized carbides in a matrix of ferrite can be obtained by full annealing. Such a structure is not as well suited for machining as a normalized structure and the steel tends to tear, chips break away with difficulty, and metal tends to build up on the machining tool. However, the soft and ductile spheroidized structure is preferred for severe cold-forming operations such as spinning and deep drawing.
Medium C low alloy steels are cut, sheared, punched, and cold formed in the annealed condition. Cutting is normally done with a saw or abrasive disk. If these steels are flame cut, most of them are preheated to around 315 deg C. After flame cutting, since the cut edge is hard, blanks are annealed before being formed or machined. Preferably, medium C low alloy steels are welded in the annealed or normalized condition and then heat treated to the desired strength. Such processes as inert gas tungsten arc, shielded metal arc, inert gas metal arc, and pressure processes, as well as flash welding, can be used. Filler wire having the same composition as the base steel is preferred, but if such is not available, the filler wire is at least to be of a composition which produces a deposit that responds to the heat treatment in around the same manner as the base steel. To avoid brittleness and cracking, preheating and inter-pass heating are used, and complex structures are stress relieved or hardened and tempered immediately after welding.
Grade 4130 steel – Grade 4130 steel is a water hardening alloy steel of low-to-intermediate hardenability. It retains good tensile, fatigue, and impact properties upto around 370 deg C. However, it has poor impact properties at cryogenic temperatures. This steel is not subject to temper embrittlement and can be nitrided. It is normally forged at 1,100 deg C to 1,200 deg C. Finishing temperature is not to fall below 980 deg C. This steel is available in the form of billet, bar, rod, forgings, sheet, plate, pipe, and castings. Grade 4130 steel is used to make automotive connecting rods, engine mounting lugs, shafts, fittings, bushings, gears, bolts, axles, gas cylinders, air-frame components, hydraulic lines, and nitrided machinery parts. The standard heat treatments which apply to grade 4130 steel are as follows
- Normalizing – The steel is heated to 870 deg C to 925 deg C and held for a time period which depends on section thickness. It is then air cooled. Tempering at 480 deg C or above is frequently done after normalizing to increase the yield strength.
- Annealing – The steel is heated to 830 deg C to 860 deg C and held for a time period which depends on section thickness or furnace load. The steel is furnace cooled.
- Hardening – The steel is heated to 845 deg C to 870 deg C, held, and then water quenched or the steel is heated heat to 860 deg C to 885 deg C, held, and then oil quenched. Holding time depends on section thickness.
- Tempering – The steel is held at least 2 hours at 200 deg C to 700 deg C, air cooled or water quenched. Tempering temperature and time at temperature depend mainly on desired hardness or strength level.
- Spheroidizing – The steel is heated to 760 deg C to 775 deg C and is held for 6 hours to 12 hours and then cooled slowly.
Since the steel grade has low hardenability, section thickness is to be considered when heat treating to high strength.
Grade 4140 steel – Grade 4140 steel is similar in composition to grade 4130 steel except for a higher C content. It is used in applications requiring a combination of reasonable hardenability and good strength and toughness, but in which service conditions are only fairly severe. Since it has higher C content, grade 4140 steel has greater hardenability and strength than the grade 4130 steel has, but with some sacrifice in formability and weldability. Tensile strengths of upto 1,650 MPa are readily achieved in grade 4140 steel through conventional quenching and tempering heat treatments. This steel can be used at temperatures as high as 480 deg C, above which its strength decreases rapidly with increasing temperature. The material can be readily nitrided. Like other martensitic and ferritic steels, 4140 undergoes a transition from ductile to brittle behaviour at low temperatures, the transition temperature varying with heat treatment and stress concentration.
When 4140 is heat treated to high strength levels, it is subject to H2 embrittlement, such as that resulting from acid pickling or from Cr or Cd (cadmium) electroplating. Ductility can be restored by baking for 2 hours to 4 hours at 190 deg C. The forging of grade 4140 steel can be done readily, normally at 1,100 deg C to 1,200 deg C, with the finishing temperature not to go below 980 deg C. Parts are to be cooled slowly after hot forming.
This steel has good weldability using any of the standard welding methods. For welding, pre-heating at 150 deg C to 260 deg C and post-heating at 600 deg C to 675 deg C, followed by slow cooling, are desired. Cold drawn grade 4140 steel has a machinability rating of 62 % (free cutting steel, 100 %).
The grade 4140 steel is available as bar, rod, forgings, sheet, plate, strip, and castings. It is used for several high strength machine parts (some of them nitrided) such as connecting rods, crank-shafts, steering knuckles, axles, oil well drilling bits, piston rods, pump parts, high-pressure pipe, large industrial gears, flanges, collets, machine tool parts, wrenches, tong jaws, sprockets, and studs. The standard heat treatments which apply to grade 4140 steel are as follows.
- Normalizing – The steel is heated to 845 deg C to 900 deg C and held for a time period which depends on section thickness. It is then air cooled.
- Annealing – The steel is heated to 845 deg C to 870 deg C and held for a time period which depends on section thickness or furnace load. The steel is furnace cooled.
- Hardening – The steel is heated to 830 deg C to 870 deg C, held, and then oil quenched (for water quenching, which is rarely used, hardening temperatures are 815 deg C to 845 deg C). Holding time depends on section thickness.
- Tempering – The steel is held at least 0.5 hours at 190 deg C to 230 deg C, or 370 deg C to 675 deg C and then air cooled or water quenched. Tempering temperature and time at temperature depend mainly on the desired hardness. To avoid blue brittleness, grade 4140 steel is normally not tempered between 230 deg C and 370 deg C. Grade 4140 steel is not subject to temper embrittlement.
- Spheroidizing – The steel is heated to 760 deg C to 775 deg C and is held 6 hours to 12 hours and then cooled slowly.
Since grade 4140 steel is not deep-hardening steel, section size is to be considered when specifying the heat treatment, especially for high strength levels. There are effects of mass on the hardness and tensile properties. As expected, grade 4140 steel has low impact strength at cryogenic temperatures.\
Grade 4340 steel – Grade 4340 steel is considered the standard by which other ultra-high strength steels are compared. It combines deep hardenability with high ductility, toughness, and strength. It has high fatigue and creep resistance. It is frequently used where severe service conditions exist and where high strength in heavy sections is needed. In thin sections, this steel is air hardening, however, in practice, it is normally oil quenched. It is especially resistant to temper embrittlement. It does not soften readily at high temperatures and this means that it shows good retention of strength.
H2 embrittlement is a problem for grade 4340 steel heat treated to tensile strengths higher than around 1,400 MPa. Parts exposed to H2, such as during pickling and plating, are to be baked subsequently. This steel shows extremely poor resistance to stress-corrosion cracking when tempered to tensile strengths of 1,500 MPa to 1,950 MPa. It can be readily nitrided, which frequently improves fatigue life.
The 4340 steel is normally forged at 1,065 deg C to 1,230 deg C. After forging, parts can be air cooled in a dry place or, preferably, furnace cooled. The machinability rating of grade 4340 steel is 55 % for cold-drawn material, and 45 % for annealed material (cold-rolled free cutting steel being 100 %). A partly spheroidized structure obtained by normalizing and then tempering at 650 deg C is best of optimum machinability.
The grade 4340 steel has good welding characteristics. It can be readily gas or arc welded, but welding rods of the same composition are to be used. Since grade 4340 steel is air hardening, welded parts are to be either annealed or normalized and tempered shortly after welding. This steel is widely and readily available in the form of billet, bar, rod, forgings, sheet, pipe, and welding wire. It is also produced as light plate and castings. Typical applications include bolts, screws, and other fasteners, gears, pinions, shafts, and similar machinery components, crank-shafts and piston rods for engines, and landing gear and other critical structural members for aircraft. The standard heat treatments which apply to grade 4340 steel are as follows.
- Normalizing – The steel is heated to 845 deg C to 900 deg C and held for a time period which depends on section thickness. It is then air cooled.
- Annealing – The steel is heated to 830 deg C to 860 deg C and held for a time period which depends on section thickness or furnace load. The steel is furnace cooled.
- Hardening – The steel is heated to 800 deg C to 845 deg C and held for 15 minutes for each 25 mm of thickness (15 minutes minimum) then it is either oil quenched to below 65 deg C, or quenched in fused salt at 200 deg C to 210 deg C and then held for 10 minutes, and after that air cooled to below 65 deg C.
- Tempering – The steel is held at least 0.5 hours at 200 deg C to 650 deg C, and then air cooled. Temperature and time depend mainly on desired final hardness.
- Spheroidizing – The preferred schedule is to preheat to 690 deg C and hold the steel for 2 hours, then increase the temperature to 745 deg C and hold for 2 hours, cool the steel to 650 deg C and hold for 6 hours, then furnace cool to around 600 deg C, and finally air cool to room temperature. An alternative schedule is to heat to 730 deg C to 745 deg C, hold for several hours, and then furnace cool to room temperature.
- Stress relieving – After straightening, forming, or machining, parts can be stress relieved at 650 deg C to 675 deg C.
- Baking – For avoiding H2 embrittlement, plated parts are to be baked for at least 8 hours at 185 deg C to 195 deg C as soon after plating as possible.
Through-hardening of grade 4340 steel can be done by oil quenching, for round sections upto 75 mm in diameter, and by water quenching, for larger sections (upto the limit of hardenability). Section size has an influence on the tensile properties of oil-quenched and water-quenched grade 4340 steel.
Variation in hardness of 4340 steel with the tempering temperature is shown in Fig 2. All samples are oil quenched from 845 deg C and tempered for 2 hours at temperature. AQ is as quenched. Fig 2 also shows variations in tensile properties with test temperature with properties determined using samples heat treated to a room-temperature tensile strength of 1,380 MPa. In Fig 2, low-temperature tensile properties of 4340 steel are shown with properties determined for samples oil quenched from 860 deg C, and double tempered at 230 deg C.
Fig 2 Variation in the properties of grade 4340 steel
Consumable electrode vacuum melting (normally known as vacuum arc remelting, or VAR) and electroslag remelting (ESR) have resulted in considerable improvements in the properties of grade 4340 steel because of significant reductions in gas content and in the size and number of non-metallic inclusions. Typical average gas contents of air-melted grade 4340 steel are 1.4 ppm H2, 25 ppm O2, and 100 ppm N2 which can be reduced to around 0.9 ppm H2, 4 ppm O2, and 53 ppm N2 by vacuum arc remelting. The remelted steels are more homogeneous than are the air-melted steel products. Mechanical properties are considerably improved by VAR processing, especially in the transverse direction, although for plane-strain fracture toughness there appear to be no significant differences between longitudinal and transverse values for the same heat. There is no significant differences in properties between VAR and ESR treated steels and the two processes appear to give roughly equivalent improvements over air melting process.
Grade 300M steel – Alloy steel 300M is basically Si-modified (1.6 % Si) grade 4340 steel, but it has slightly higher C and Mo contents and also contains V. This steel shows deep hardenability and has ductility and toughness at tensile strengths of 1,860 to 2,070 MPa. Many of the properties of this steel are similar to those of grade 4340 steel, except that the increased Si content provides deeper hardenability, increased solid-solution strengthening, and better resistance to softening at high temperatures.
Compared to grade 4340 steel of similar strength, 300M steel can be tempered at a higher temperature, which provides greater relief of quenching stresses. The so-called 260 deg C embrittlement is displaced to higher temperatures. Because of the high Si and Mo contents, 300M steel is particularly prone to decarburization. During thermal processing, care is to be taken to avoid decarburization, or the decarburized layer is to be removed after processing. When heat treated to strength levels higher than 1,380 MPa, 300M steel is susceptible to H2 embrittlement. If the steel is properly baked after plating, the resulting improvement in properties is better than that for grade 4340 or D-6ac steel of equal strength. The 300M steel is forged at 1,065 deg C to 1,095 deg C. Forging is not to be done below 925 deg C. After forging, it is preferred that the parts are slowly cooled in a furnace, but they can be allowed to cool in air in a dry place.
Although 300M steel can be readily gas or arc welded, welding is generally not desired. Welding rod of the same composition is to be used. Because 300M steel is an air-hardening steel, parts are to be either annealed or normalized and tempered after welding. The machinability rating of annealed 300M steel is around 45 % (free cutting steel, 100 %). A partially spheroidized structure, achieved by normalizing and then tempering at 650 deg C to 675 deg C, gives optimum machinability. Typical applications of 300M steel, which is available as bar, sheet, plate, wire, pipe, forgings, and castings, are aircraft landing gear, airframe parts, fasteners, and pressure vessels. The standard heat treatments which apply to 300M steel are given below.
- Normalizing – The steel is heated to 915 deg C to 940 deg C and held for a time period which depends on section thickness. It is then air cooled. If normalizing is done to improve machinability, then it is charged into a tempering furnace at 650 deg C to 675 deg C before the steel reaches room temperature.
- Hardening – The steel is austenitized at 860 deg C to 885 deg C and oil quenching is done to below 70 deg C or quenching is done in salt at 200 deg C to 210 deg C. The steel is held for 10 minutes, and then air cooled to 70 deg C or below.
- Tempering – The steel is held for 2 hours to 4 hours at 260 deg C to 315 deg C. Double tempering is normally desired. This tempering procedure produces the best combination of high yield strength and high impact properties. Tempering outside this temperature range results in severe deterioration of properties
- Spheroidizing – The steel is heated to around 775 deg C and held for a time period which depends on section thickness or furnace load. It is then cooled to 650 deg C at a rate less than 5.5 deg C per hour and then cooled to 480 deg C less than 10 deg C per hour and finally, air cooled to room temperature. The same schedule is recommended for annealing.
There are variations in hardness and mechanical properties of 300M steel with tempering temperature. Since 300M steel has deep hardenability, heat-treated bars of 75 mm in diameter have essentially the same tensile properties which the bars of 25 mm in diameter have. Reductions in tensile ductility and impact strength, however, are observed in heat-treated bars of 145 mm in diameter.
In fatigue tests of polished samples, endurance limits of around 800 MPa and 585 MPa are found for longitudinal and transverse samples, respectively, of air-melted 300M steel heat treated to a tensile strength of around 2,025 MPa by oil quenching from 870 deg C and tempering at 290 deg C. VAR and ESR improve transverse ductility and impact strength by producing a cleaner micro-structure.
In one study, transverse tensile properties have been determined for samples of 300M steel taken from 40 billets, each 125 mm square, representing 7 VAR heats. Samples have been normalized for 1 hour at 925 deg C, reheated for 1 hour at 870 deg C and then oil quenched and tempered for 4 hours at 315 deg C. Average properties which have been found are tensile strength – 1,978 MPa, yield strength – 1,671 MPa, elongation – 9.3 %, and reduction in area – 36.6 %. Property ranges have been tensile strength – 1,896 to 2,039 MPa, yield strength – 1,581 to 1,752 MPa, elongation – 9 % to 10 %, and reduction in area – 31.7 % to 45 %.
Fatigue test results on transverse samples indicate fatigue limits of around 580 MPa for air-melted 300M steel and around 675 MPa for VAR treated 300M steel which the latter represents an improvement of around 17 % over air-melted 300M steel. In the same study, it has been seen that there is no significant difference in transverse fatigue limit between VAR treated 300M steel and ESR treated 300M steel which had been hot reduced only 75 % from the ingot. In all these studies, remelted steels (whether VAR or ESR) has shown better ductility and toughness, particularly in the transverse direction, although there does not appear to be any significant difference between longitudinal and transverse plane-strain fracture toughness values.
D-6a and D-6ac Steel – D-6a is a low-alloy ultra-high strength steel developed for aircraft and missile structural applications. It is designed primarily for use at room-temperature tensile strengths of 1,800 MPa to 2,000 MPa. D-6a steel maintains a very high ratio of yield strength to tensile strength upto a tensile strength of 1,930 MPa, combined with good ductility. It has good notch toughness, which results in high resistance to impact loading. It is deeper hardening steel than grade 4340 steel and does not show temper embrittlement. It retains high strength at high temperature. Susceptibility of D-6a steel to stress-corrosion cracking and corrosion fatigue in moist and aqueous environments is comparable to that of 300M steel at the same strength level.
The alloy steel is called D-6a when produced by air melting in an electric furnace and D-6ac when produced by air melting followed by VAR. The mechanical properties of D-6a and D-6ac differ somewhat as a result of the differences in melting practices. Other characteristics of the two steels, including processing behaviour, are identical. D-6a and D-6ac are available as bar, rod, billet, and forgings and can be made as flat-rolled products (sheet and plate) as well. These forms are used in landing gear and critical structural components for aircraft, motor cases for solid-fuel rockets, shafts, gears, springs, dies, dummy blocks, and backer blocks.
For forging D-6a steel, it is to be heated to a maximum temperature of 1,230 deg C. Forging is to be finished above 980 deg C. Finished forging is to be cooled slowly, either in a furnace or embedded in a suitable insulating medium such as ashes or lime. For maximum machinability, the parts are to be charged into a 650 deg C furnace immediately after forging and held for 12 hours then the temperature is to be increased to 900 deg C and held for a time period which depends on section size. After this, parts are to be cooled to 650 deg C, held 10 hours, and finally air cooled to room temperature.
The material D-6a, even in heavy sections, can be welded provided that the techniques and controls normally employed for welding medium carbon, high hardenability alloy steels are used. Welding rod of the same composition is to be used. For critical applications, gas tungsten arc welding is preferred. Filler metal wire is to be of vacuum-melted containing less C than is in the base steel and minimum amounts of phosphorus (P), sulphur (S), and dissolved gases. Welds made in this manner have higher toughness than that of the base steel, but slightly lower strength. Pre-heat and inter-pass temperatures of 230 deg C to 290 deg C are desired. Highly restrained weldments are to be post-heated for 1.5 hours at 300 deg C to 330 deg C and cooled in still air. When the weldments achieve 150 deg C, they are to be charged immediately into a furnace for stress relieving at 650 deg C to 700 deg C. Annealed D-6a has a machinability rating of 50 % to 55 % (free cutting steel, 100 %). When the steel is to be severely cold formed, it is usually normalized and then spheroidized before working. The standard heat treatments which apply to D-6a and D-6ac steels are given below.
- Normalizing –The steel is heated to 870 deg C to 955 deg C and held for a time period which depends on section thickness. It is then air cooled.
- Annealing – The steel is heated to 815 deg C to 845 deg C and held for a time period which depends on section thickness or furnace load, The steel is furnace cooled to 540 deg C at a rate not higher than 28 deg C per hour and then air cooled to room temperature.
- Hardening – The steel is austenitize at 845 deg C to 900 deg C for 0.5 hours to 2 hours. Sections not thicker than 25 mm can be air cooled. Sections thicker than 25 mm can be oil quenched to 65 deg C or salt quenched to 205 deg C and then air cooled. For optimum dimensional stability, ‘aus-bay’ quenching is done in a furnace at 525 deg C, the temperature is equalized, and then quenching is done in an oil bath held at 60 deg C or quenching in 250 deg C salt and air cooled. The cooling rate during quenching significantly affects fracture toughness. For high fracture toughness, especially in heavy sections, austenitizing is done at 925 deg C, aus-bay quenching to 525 deg C, and equalizing and oil quenching is done to 60 deg C.
- Tempering – Immediately after hardening, the steel is held for 2 hours to 4 hours in the range of 200 deg C to 700 deg C, depending on desired strength or hardness.
- Spheroidizing – The steel is heated to 730 deg C and held for 5 hours and then the temperature is increased to 760 deg C and held for 1 hour. The furnace is cooled to 690 deg C and held for 10 hours. The furnace is then cooled to 650 deg C and held for 8 hours. Then the steel is air cooled to room temperature.
- Stress relieving – The steel is heated to an appropriate temperature in the range of 540 deg C to 675 deg C and held for 1 hour to 2 hours and then air cooled.
The effect of tempering temperature on typical room-temperature hardness of D-6a steel bar is shown in Fig 3. D-6a maintains impact resistance to very low temperatures (Fig 3). Data on stress rupture life at 480 deg C and 540 deg C are shown in Fig 3. The effects of tempering temperature on smooth bar and notched-bar tensile strengths are plotted in Fig 3. The rate of cooling during quenching has a significant effect on fracture toughness. This steel grade is susceptible to stress-corrosion cracking. The steel has high fatigue strength.
Fig 3 variations in the properties of D-6a steels
Grade 6150 steel – Grade 6150 steel is a tough, shock-resisting, shallow-hardening Cr-V steel with high fatigue and impact resistance in the heat-treated condition. It can be nitrided for maximum surface hardness and abrasion resistance. Nitriding characteristics are similar to those of grade 4140 and grade 4340 steels. The grade 6150 steel can be forged from temperatures upto 1,200 deg C, but the normal temperature range is 1,175 deg C to 950 deg C. Parts made of grade 6150 steel can readily be welded using any of the standard welding methods. After welding, parts are to be normalized and then hardened and tempered to the desired hardness.
For best machinability, grade 6150 steel is to be in the annealed condition. Machinability rating is around 55 % (free cutting steel 100 %). The optimum micro-structure for machining is coarse lamellar pearlite and / or coarse spheroidite as in the case of with other low-alloy steels of around the same C content. Chips are continuous and springy, which can make the steel difficult to machine. It is available as bar, rod, plate, sheet, strip, wire, and pipe. Grade 6150 steel can be forged or cast into shapes. Typical applications include gears, pinions, springs (both coiled and flat), shafts, axles, heavy duty pins, bolts, and machinery parts. The heat treatments which apply to grade 6150 steel are as follows.
- Normalizing – The steel is heated to 870 deg C to 955 deg C and held for a time period which depends on section thickness and then it is air cooled.
- Annealing – The steel is heated to 845 deg C to 900 deg C and held for a time period which depends on section thickness or furnace load. The steel is then furnace cooled.
- Hardening – The steel is austenitized at 845 deg C to 900 deg C and then it is oil quenched.
- Tempering – The steel is held at least 0.5 hours at 200 deg C to 650 deg C. Tempering temperature and time at temperature primarily depend on desired final hardness.
- Austempering – The steel is austenitized in a salt bath at 845 deg C to 900 deg C. It is then quenched in a salt bath at 230 deg C to 315 deg C and held for 20 minutes to 30 minutes, and quenched or air cooled to room temperature.
- Martempering – It is austenitized in a salt bath at 845 deg C to 870 deg C and then quenched in a salt bath at 230 deg C to 260 deg C, equalized, and then air cooled or quenched to room temperature. Tempering is done to the desired hardness.
- Spheroidizing – The steel is heated to 800 deg C to 830 deg C and held until heated through. It is then furnace cooled to 650 deg C, and held for several hours and then cooled slowly to room temperature.
Grade 8640 steel – Grade 8640 steel has been especially designed to provide the maximum hardenability and best combination of properties possible with minimum alloying additions. The 8640 steel is oil hardening steel, but can be water hardened if precautions are taken to prevent cracking. It shows properties similar to those of grade of 4340 steel, except that its strength in large sections is not as high. This steel is available as billets, bars, rods, forgings and castings. It is used to make gears, pinions, shafts, axles, studs, fasteners, machinery parts, and forged hand tools.
Grade 8640 steel can be forged at temperatures upto 1,200 deg C, but is normally forged in the range from 1,175 deg C to 950 deg C. Forged parts are cooled slowly from the forging temperature and then annealed prior to machining. This steel can be welded by any of the standard welding methods. Since 8640 steel has some air-hardening tendencies, pre-heating to 150 deg C to 260 deg C before welding and post-heating after welding are desired. Stress relieving at 600 deg C to 650 deg C is quite satisfactory for most welded parts. Cold-drawn 8640 steel has a machinability rating of 64 % (free cutting steel 100 %). Annealing prior to cold drawing can improve machinability by around 10 %. The heat treatments which apply to grade 8640 steel are as follows.
- Normalizing – The steel is heated to 870 deg C to 925 deg C and is held for a time period which depends on section thickness and then it is air cooled.
- Annealing – The steel is heated to 845 deg C to 870 deg C and held for a time period which depends on section thickness or furnace load and is then furnace cooled.
- Hardening – The steel is austenitized at 815 deg C to 845 deg C and then quenched in oil or water.
- Tempering – The steel is held at least 0.5 hours at 200 deg C to 650 deg C.
- Spheroidizing – The steel is heated to 705 deg C to 720 C and held several hours and then it is furnace cooled.
Medium alloy air hardening steels
The ultra-high strength steels (grades H11 modified and H13), are popularly known as 5 % Cr hot work die steels. Besides being extensively used in dies, these steels are widely used for structural applications, but not as widely as they were earlier, mainly because of the development of several other steels at essentially the same cost but with substantially greater fracture toughness at equivalent strength. However, H11 modified and H13 possess some attractive features. Both can be hardened through in large sections by air cooling.
Grade H11 modified – This steel is a modification of the martensitic hot-work die steel grade H11, the significant difference being a slightly higher C content. The H11 modified steel can be heat treated to strengths higher than 2,070 MPa. It is air hardened, which results in minimal residual stress after hardening. Because H11 modified steel is a secondary hardening steel, it develops optimum properties when tempered at temperatures above 510 deg C. The high tempering temperatures used for this steel provide substantial stress relief and stabilization of properties so that the material can be used to advantage at high temperatures. This also enables heat treated parts to be warm worked at temperatures as much as 55 deg C below the prior tempering temperature or to be pre-heated for welding. At high strength levels (those exceeding 1,800 MPa), H11 modified steel has good ductility, impact strength, notch toughness, and fatigue life, as well as high creep and rupture strength, at temperatures upto around 650 deg C.
It is used for parts requiring maximum levels of strength, ductility, toughness, fatigue resistance, and thermal stability at temperatures between -75 deg C and 540 deg C. At high temperatures, parts are to be protected from corrosion (oxidation) by an appropriate surface treatment. The H11 modified steel has good formability in the annealed condition and is readily welded. It is subject to H2 embrittlement. Its fracture toughness is rather low. If it is used in critical applications at yield strengths above 1,380 MP, care is to be taken to eliminate small discontinuities.
The H11 modified steel is available as bar, billet, rod, wire, plate, sheet, strip, forgings, and extrusions. It is used for parts requiring high strength combined with either strength retention at high temperatures or moderate corrosion resistance. Typical applications include aircraft landing gear components, airframe components, internal parts for steam and gas turbines, fasteners, springs, and hot-work dies. Parts to be used at high temperatures are normally protected by Ni-Cd plating. If such plating is done, baking to avoid H2 induced delayed cracking is desired. Alternatively, part surfaces can be protected from oxidation by hot dipping in aluminum (Al) or by applying a heat-resistant paint.
The grade H11 modified steel is readily forged from 1,120 deg C to 1,150 deg C. Preferably, the steel is to be preheated at 790 deg C to 815 deg C and then heated uniformly to the forging temperature. Forging is not to be continued below 925 deg C. Steel can be reheated as frequently as necessary. Since H11 modified is air hardening, it is to be cooled slowly after forging to prevent stress cracks. After forging, the part is to be charged into a furnace at around 790 deg C and then soaked until the temperature is uniform. It is then slowly cooled, either while retained in the furnace or buried in an insulating medium such as lime, mica, or a siliceous filler material such as silocel. When the forging has cooled, it is to be annealed.
The steel H11 modified steel, even in heavy sections, is readily welded. Fusion welding normally is done with an inert-gas process or with coated electrodes. Filler metal is to be of the same general composition. Preheating at around 540 deg C is desired, and during welding the temperature is to be maintained above 315 deg C. Thin sheet can be welded without pre-heating, but is to be post-heated at around 760 deg C. Weldments, especially heavy section weldments, are to be cooled slowly in a furnace or in an insulating medium. All weldments are to be fully annealed after welding. Weldments of H11 modified steel have shown weld metal strength and ductility equal to or greater than those of the base steel. The machinability rating for H11 modified steel is around 60 % of the rating for 1 % C steel, or around 45 % of that for free cutting steel. The standard heat treatments which apply to H11 modified steel are as follows.
- Normalizing – It is normally not necessary. For effective homogenization, the steel is heated to around 1,065 deg C, soaked for 1 hour for each 25 mm of thickness and then air cooled. The steel is then annealed immediately after the part reaches room temperature. There is a possibility that H11 modified steel can crack during this treatment.
- Annealing – The steel is heated to 845 deg C to 885 deg C and held to equalize the temperature and then cooled very slowly in the furnace to around 480 deg C and then more rapidly to room temperature. This treatment is to produce a fully spheroidized microstructure free of grain boundary carbide networks.
- Hardening – The steel is pre-heated to 760 deg C to 815 deg C and then the temperature is raised to 995 deg C to 1,025 °C and held for 20 minutes plus 5 minutes for each 25 mm of thickness. After this, the steel is air cooled. For a few applications, oil quenching from the low end of the hardening temperature range can be done. Air cooling, which produces less distortion than oil quenching, is normally used more.
- Tempering – The steel is heated at the secondary hardening temperature of around 510 deg C for maximum hardness and strength, or above the secondary hardening peak to temper back to a lower hardness or strength. A minimum of 1 hour at temperature is to be allowed, but preferably parts are to be double tempered. The parts are held for 2 hours at temperature, cooled to room temperature, and then held for 2 hours more at temperature. Triple tempering is more desirable, especially for critical parts. For high-temperature applications, parts are to be tempered at a temperature above the maximum service temperature to guard against unwanted changes in properties during service.
- Stress relieving – The steel is heated to 650 deg C to 675 deg C, and cooled slowly to room temperature. This treatment is frequently used to achieve greater dimensional accuracy in heat-treated parts by stress relieving rough-machined parts, then finish machining, and finally heat treating to the desired hardness ·
- Nitriding – Finish-machined and heat-treated parts are normally gas or liquid nitrided at temperatures of around 525 deg C. The depth of the nitrided case depends on time at temperature. For example, gas nitriding in 20 % to 30 % dissociated ammonia for 8 hour to 48 hour normally produces a case depth of around 0.2 mm to 0.35 mm.
- Baking – After plating in an acid bath, or after other processing which can introduce H2 into the steel, parts are to be baked for 24 hours or longer at 190 deg C or above.
This steel is quite notch-tough. The ratio of notched-bar tensile strength to smooth-bar tensile strength at room temperature is around 1.4 at a smooth-bar tensile strength of 1,380 MPa and around 1.15 at strength of 1,930 MPa. Because of secondary hardening characteristics, H11 modified steel has good temper resistance, which results in high hardness and strength at high temperatures. Regardless of the initial room-temperature hardness, the hot hardness drops steeply to levels corresponding to the annealed state at temperatures above 620 deg C.
The steel H11 modified has comparatively low fracture toughness, but has good resistance to stress-corrosion cracking compared to other ultra-high strength steels heat treated to the same strength. Considerable improvements in H11 modified properties such as transverse ductility and toughness, particularly in large sections, can be achieved by VAR and ESR, both of which result in greater homogeneity and cleanliness of the steel.
Grade H13 steel – Grade H13 steel is a 5 % Cr ultra-high strength steel similar to H11 modified steel in composition, heat treatment, and many properties. The main difference in composition is the higher content of V in H13. This leads to higher dispersion of hard V carbides, which results in higher wear resistance. Also, H13 steel has a slightly wider range of C content than does H11 modified steel. The C content of H13 steel can be near the high or low side of the accepted range, with a corresponding variation in strength and ductility for a given heat treatment.
Like H11 modified steel, H13 steel is a secondary hardening steel. It has good temper resistance and maintains high hardness and strength at higher temperatures. It is deep hardening, which allows large sections to be hardened by air cooling. H13 steel can be heat treated to strengths exceeding 2,070 MPa. Like H11 modified steel, it has good ductility and impact strength. With standard heat treatment, the fracture toughness of H13 steel appears to be even lower than that of H11 modified steel.
The H13 steel has good resistance to thermal fatigue. Hot-work tooling made from H13 steel can be safely water cooled between hot-working operations. Its resistance to thermal fatigue, erosion, and wear has made it a preferred die material for Al and Mg (magnesium) die casting, as well as for many other hot-work applications. However, H13 steel is subject to H2 embrittlement. It can be nitrided for additional wear resistance.
Although H13 steel is not being used as widely as H11 modified steel as an ultra-high strength constructional steel, the similarities in the properties make H13 steel equally attractive for such applications. This is mainly true in non critical application in which slightly higher wear resistance is an advantage. The H13 steel is available as bar, rod, billet, and forgings. Typical hot-work applications include die casting dies, inserts, cores, ejector pins, plungers, sleeves, slides, forging dies, extrusion dies, dummy blocks, and mandrels. Other tooling and structural applications include punches, shafts, beams, torsion bars, shrouds, and ratchets.
For forging, H13 steel is heated slowly and uniformly to a temperature of 1,090 deg C to 1,150 deg C, preferably after preheating at 760 deg C to 815 deg C. The steel is to be thoroughly heated before forging. Forging is not to be done below 900 deg C, but the parts can be reheated as frequently as necessary. Since H13 steel is air hardening, parts are to be cooled slowly after forging. Simple forgings can be cooled in an insulating medium such as dry ashes, lime, or expanded mica. The best practice for large forgings is to place in a heated furnace at around 790 deg C, soak until the temperature is uniform, shut off the furnace, and let cool slowly. Parts are then to be given a full spheroidizing annealing.
When annealed H13 steel is welded, it is to be preheated to 540 deg C, or to as high a temperature as is practical, preferably in a furnace to ensure uniform, stress-free preheating. Uncoated rod, preferably of the same general composition, is to be used, with shielded-arc equipment. The temperature of the part is to be kept above 315 deg C, and the part is reheated if necessary, until welding has been completed. After welding, the part is to be cooled slowly in an insulating medium and given a full anneal.
A heat-treated part such as a die can be welded using the same procedure, preferably preheating the part in a furnace to a temperature around 55 deg C below the original tempering temperature. After welding, the part is to be placed in a furnace at the preheating temperature and cooled slowly to room temperature. It is desired that the part then be reheated to just below the original tempering temperature and air cooled. This helps to relieve welding stresses and blend the hardness of the weld area into that of the base steel. Regardless of the situation, adequate preheating and slow cooling are essential to minimize the risk of cracking during welding. Fully annealed H13 steel has a machinability rating which is around 70 % of the rating for 1 % C tool steel, or around 45 % of that of free cutting steel. The heat treatments which apply to H13 steel are as follows.
- Normalizing – Normalizing is not advised for H13 steel. Some improvement in homogeneity can be achieved by preheating to around 790 deg C, heating slowly and uniformly to 1,040 deg C to 1,065 deg C, holding for 1 hour for each 25 mm of thickness, and then air cooling. Just before the part reaches room temperature, it is to be recharged into a furnace and given a full anneal. There is a risk of cracking during this treatment, especially if done in a furnace in which the atmosphere is not controlled to prevent surface decarburization.
- Annealing – The steel is heated uniformly to 860 deg C to 900 deg C in a furnace with controlled atmosphere, or with the part packed in a neutral compound, so that decarburization is prevented. The steel is cooled very slowly in the furnace to around 480 deg C and then cooled more rapidly to room temperature. This treatment results in a fully spheroidized micro-structure.
- Hardening – The steel is heated slowly and uniformly to 995 deg C to 1,025 deg C and soaked for 20 minutes plus 5 minutes for each 25 mm of thickness, Pre-heating at 790 deg C to 815 deg C is normally desired for thick parts. The steel is air cooled in still air. Air cooling is normally done from the high side of the hardening temperature range. For a few applications, H13 steel can be oil quenched from the low side of the hardening temperature, but there is a risk of distortion or cracking.
- Tempering – The steel is heated at the secondary hardening peak of around 510 deg C for maximum hardness and strength, or at higher temperatures to temper back to a lower level of hardness or strength. Double tempering consisting of 2 hours at temperature, air cooling, then 2 hours more at temperature, is desired. Occasionally, triple tempering can be desirable.
- Stress relieving – The steel is heated to 650 deg C to 675 deg C and is soaked for 1 hour or more, and cooled slowly to room temperature. This treatment is frequently used to achieve greater dimensional accuracy in heat-treated parts by stress relieving rough-machined parts, then finish machining, and finally heat treating to the desired hardness.
- Nitriding – Finish-machined and heat-treated parts can be nitrided to produce a highly wear-resistant surface. Since it is carried out at the normal tempering temperature, nitriding can serve as the second temper in a double-tempering treatment. The depth of the nitrided case depends on the time at temperature. For example, gas nitriding at 510 deg C for 10 hour to 12 hour produces a case depth of 0.1 mm to 0.13 mm and for 40 hours to 50 hours produces a case depth of about 0.3 mm to 0.4 mm. Parts which have been deep nitrided are normally lapped or gently surface ground to remove the thin, brittle white layer. Selective nitriding is sometimes done to produce a nitrided case only where it is needed. Copper (Cu) plating is preferred for stopping off areas which are not to be nitrided. Stop-offs containing lead (Pb) is to be avoided, since Pb embrittles H13 steel.
The properties normally specified for H13 steel are with a C content in the middle of the composition range. Somewhat different properties are to be expected when the C content is near either the high end or the low end of the range. Since it is a secondary hardening steel, H13 steel maintains high hardness and strength at high temperatures. Regardless of the initial room-temperature hardness, H13 steel attains basically the same low properties at 650 deg C. Like H11 modified steel, H13 steel has good impact properties at temperatures upto 540 deg C. Little work has been done to determine the fracture toughness of H13 steels. It appears that with the standard heat treatment H13 has slightly lower fracture toughness than H11 modified steel.
H13 steels produced by VAR or ESR processes have better cleanliness and homogeneity than air melted H13 steels. The VAR and ESR processes are used to produce steels with superior ductility, impact strength, and fatigue resistance, especially in the transverse direction, and particularly in large section sizes. In axial (tension-tension) fatigue tests, the life of ESR H13 steel is superior to that of air-melted H13 steel. When both are fatigue tested under fully reversed stresses, there is no significant difference in the life of longitudinal samples. However, for transverse samples, the life of ESR H13 steel is considerably better.
High fracture toughness steels
High-strength, high fracture toughness steels are normally commercial structural steels capable of a yield strength of 1,380 MPa. These steels also show stress corrosion cracking resistance. There are also a number of developmental steels which are not fully commercial steels. The HP-9-4-30 grade steel and AF1410 grade steel, however, are described below. Both these steels are of the Ni-Co-Fe type and have a number of similar characteristics. Both are weldable, and the melt practice needs a minimum of VAR. Control of residual elements to low levels is needed for optimum toughness. The machining practices used for grade 4340 steel are normally satisfactory for these steels. However, Ni-Co-Fe steels are considered more difficult to machine than other alloy steels.
HP-9-4-30 steel – During the 1960s, a family of 4 weldable steels were introduced, all of which had high fracture toughness when heat treated to medium / high strength levels. Only HP-9-4-30 steel has been produced in significant quantities and is comparable to the other high-strength, high fracture toughness steels. The HP-9-4-30 steel is normally electric arc melted and then vacuum arc remelted. Forging temperatures are not to exceed 1,120 deg C. The HP-9-4-30 steel is capable of developing a tensile strength of 1,520 MPa to 1,650 MPa. This steel has deep hardenability and can be fully hardened to martensite in sections upto 150 mm thick.
The HP-9-4-30 steel in the heat-treated condition can be formed by bending, rolling, or shear spinning. Heat-treated parts can be readily welded. Tungsten arc welding under inert-gas shielding is the preferred welding process. Neither post-heating nor post-weld heat treatment is needed. After welding, parts can be stress relieved at around 540 deg C for 24 hours. This is a stress-relieving treatment and has no adverse effect on the strength or toughness of either the weld metal or the base steel. The HP-9-4-30 steel is available as billet, bar, rod, plate, sheet, and strip. It has been used for aircraft structural components, pressure vessels, rotor shafts for metal forming equipment, drop hammer rods, and high-strength shock-absorbing automotive parts. The heat treatments which apply to HP 9-4-30 steel are given below.
- Normalizing – The steel is heated to 870 deg C to 925 deg C and held for 1 hour for each 25 mm of thickness with 1 hour minimum and then air cooled.
- Annealing – The steel is heated to 620 deg C and held for 24 hours and then air cooled.
- Hardening – The steel is austenitized at 830 deg C to 860 deg C and held for 1 hour for each 25 mm of thickness with 1 hour minimum. It is then water or oil quenched. The martensitic transformation is completed by refrigerating at least for 1 hour at -87 deg c to -60 deg C. It is then allowed to warm to room temperature.
- Tempering – The steel is held at 200 deg C to 600 deg C, depending on the desired final strength. Double tempering is preferred. The most widely used tempering treatment is double tempering, which consists of 2 hours at temperature, air cooling, and then 2 hours more at temperature, at a temperature ranging from 540 deg C to 580 deg C.
- Stress relieving – It is normally needed only after welding restrained sections. The steel is heated to 540 deg C and held for 24 hours and then air cooled to room temperature.
The HP 9-4-30 steels have good thermal stability, which makes them suitable for long-term service at temperatures upto at least 370 deg C. In one study, it has been reported that the fatigue crack propagation rate is not affected by temperatures upto 345 deg C.
AF1410 grade steel – The grade AF1410 steel was an outgrowth of the US Air Force sponsorship of the advanced submarine hull steels, the result of which was the development of the low C Fe-Ni-Co type alloy steels. These alloy steels had significant stress corrosion cracking resistance. By raising the C and the Co content, the ultimate tensile strength is increased to a typical 1,615 MPa. This increase in strength is achieved while maintaining the fracture toughness. This combination of strength and toughness exceeds that of other commercially available steels, and the alloy steel has been considered as a replacement for titanium in certain aircraft parts. The AF1410 steel is air hardenable in sections upto 75 mm thick.
The preferred melting practice is presently vacuum induction melting followed by vacuum arc remelting (VIM/VAR). However, initially VIM and VIM/ESR practices have been used. Melting practice needs that impurity elements be kept at very low levels to ensure high fracture toughness. Although forgeable to 1,120 deg C, at least a 40 % reduction is to be achieved below 900 deg C to attain maximum properties.
Weldability is good using a continuous wave (CW)-gas tungsten arc welding process, provided that high-purity wire is used and oxygen (O2) contamination is avoided. Information on stress-corrosion cracking is incomplete. It is produced as bar, billet, rod, plate, sheet, and strip, AF1410 has been used for aircraft structural components.
The microstructure of AF1410 consists of Fe-Ni lath martensite, precipitation on which produces the strengthening mechanism. Quenching from the austenitizing temperature produces a highly dislocated lath martensite which has a high toughness, as measured by the Charpy V-notch impact test. Aging produces a complex series of changes in the carbide structure. At around 425 deg C, Fe3C is precipitated. At 455 deg C, Fe-Cr-Mo M2C carbide is obtained, which at 480 deg C begins to produce pure Mo-Cr M2C carbide. By raising the temperature to 510 deg C, the M2C begins to coarsen and at 540 deg C M2C begins to be replaced by M6C, which has little strengthening effect.
The steel is normally austenitized and aged. The secondary hardening, which is due to the aging, produces a maximum tensile strength when aged at 480 deg C using a 5 hour aging time and a minimum of impact energy when aged at 425 deg C. When aged in the temperature range between 425 deg C and 540 deg C, the impact energy shows a maximum at around 508 deg C. At aging temperatures above 540 deg C, both the tensile strength and the impact energy decrease rather rapidly.
The steel is subject to austenite reversion during aging. At normal aging temperatures, the retained austenite is normally less than 1 % by volume. However, 540 deg C or higher produces large amounts of austenite, and this weakens the matrix of the steel. The best combination of strength and ductility results from a 510 deg C aging. The heat treatments which apply to AF1410 steel are as follows.
- Normalizing and over-aging – This is the condition the material is normally supplied for good machinability. The steel is heated between 880 deg C and 900 deg C, and held for 1 hour for each 25 mm of thickness. It is air cooled and over-aged at 675 deg C for 5 hours minimum.
- Annealing – Normally, normalizing and overaging are used to soften and stress relieve the product. A stress relief of 675 deg C can be applied to relieve mechanical stress.
- Hardening – The steel is double austenitized, first at 870 deg C to 900 deg C, held for 1 hour for each 25 mm of thickness and oil, water, or air cooled depending on section size. It is re-austenitize at 800 deg C to 815 deg C and then oil, water, or air cooled. An alternative is to single austenitize at 800 deg C to 815 deg C, held for 1 hour for each 25 mm of thickness and then oil, water, or air cooled depending on section size.
- Quenching – Air cooling from the austenitizing temperature produces tensile strength, toughness, and fatigue strength basically equal to oil or water quenching in section sizes upto 75 mm. Refrigeration treatment of -73 deg C is optional. The aim is to reduce the amount of retained austenite. There is no real evidence that such a treatment has any substantial effect on the material or the mechanical properties
- Aging – The steel is aged at 480 deg C to 510 deg C for 5 hours to 8 hours. Air cooling is normally used.
The heat treatments show some effect on the tensile and impact properties for both 15 mm and 75 mm VIM/VAR melted plate. Room-temperature tensile properties after exposure to long-term high temperatures show some degradation after 500 hours. Comparison of fatigue crack growth rate in low-humidity and in 3.5 % saltwater solution indicates the relative insensitivity of AF1410 to saltwater exposure.