Forging Quality Steels

Forging Quality Steels

Forging of steel is the process of working hot steel is carried out normally under successive blows and sometimes by continuous squeezing. The process can be an open die forging or a close die forging. Closed-die forgings, hot upset parts, and extrusions are shaped within a cavity formed by the closed dies. Justification for selecting forging in preference to other, sometimes more economical, methods of producing useful shapes is based on several considerations. Mechanical properties in wrought steels are maximized in the direction of major metal flow during working. For complex shapes, only forging affords the opportunity to direct metal flow parallel to major applied service loads and to control, within limits, the refinement of the original material structures. Forged steels products find wide applications in automobile industry, defense equipments, railways, and several manufacturing industries.

Forging quality of a metal or an alloy means that the metal or alloy has a good ‘forgeability’. The term ‘forgeability’ is defined here as the tolerance of a metal or alloy for deformation without failure, regardless of forging-pressure requirements. Majority of the metals and alloys can be classified into one of three groups namely (i) showing good forgeability, (ii) showing poor forgeability, and (iii) showing variable forgeability. All metals and metal alloys, with very few exceptions, are suitable for forging. Fig 1 shows the comparison of the forgeabilities of various metals and alloys.

Fig 1 Comparison of forgeabilities of various metals and alloys

The forging process can be of three types. The first is hot forging process in which the forging operation is normally carried out at a temperature range of around 950 deg C to 1200 deg C. The second is the warm forging process in which the forging operation is carried out below the recrystallization temperature of steel normally at temperatures ranging from 650 deg C to 750 deg C. The third is the cold forging process in which the forging is performed at room temperature and the steel material is not heated. Since high strain rates are employed during the process of forging, the qualities needed in forging steels are critical and demanding. Further, forging components demand specialized treatments necessary for imparting special properties based on the end application of the forgings. Also, since the end use of the forged steel products is normally of critical nature, a close control over all the stages of the steel product manufacturing process is required.

Steels have been forged in quantity since near the beginning of the Industrial Revolution. Despite (or perhaps because of this long history, the forging of steels is an intuitive, empirical process, and hence the information available in literature on the subject is relatively scarce. There is a large range of types of steel from which to choose to achieve the most economical production process. Various steel groups which are used for forging includes (i) mild steels, (ii) heat treating steels, (iii) case hardening steels, (iv) nitriding steels, (v) steels for flame and induction hardening, (vi) ball and roller bearing steels, (vii) high temperature steels, (viii) steels with low temperature toughness, (ix) stainless steels, and  (x) micro alloyed or AFP (advanced formed parts ) steels.

Forging quality steels can be categorised such as (i) plain carbon steels, (ii) carbon manganese steels, (iii) molybdenum manganese steels, (iv) plain chrome steels, (v) chrome manganese steels, (vi) chrome molybdenum steels, (vii) chrome nickel steels, and (vii) chrome nickel molybdenum steels.

Selection of steel for a forged component is an integral part of the design process, and acceptable performance is dependent on this choice. A thorough understanding of the end use of the finished part serves to define the needed mechanical properties, surface finish requirements, tolerance to non-metallic inclusions, and the attendant inspection methods and criteria.

Carbon, micro-alloy and alloy steel forgings account for the greatest volume of forgings for a very wide range of applications. The steel material, which is selected for a forging application, is to be one which can achieve the required physical and mechanical properties the forged product. Where steels from several groups meet performance requirements, the most economical steel, in terms of material and processing costs, is to be chosen. Carbon, micro-alloy and alloy steels are low to moderate in cost. The main cost drivers are processing and machining. The alloy steel are readily hot forged and some shapes are cold forgeable in selected alloy steels. Alloy formulation can also be governed by the product dimensions. As section sizes become progressively heavier, higher alloy levels are needed to achieve the hardenability.

The selection of steel for a forged part normally needs some compromise between opposing factors, for example, strength versus toughness, stress-corrosion resistance versus weight, manufacturing cost versus useful load-carrying ability, production cost versus maintenance cost, and the cost of the steel raw material versus the total manufacturing cost of the forging. Selection of steel material also involves consideration of steelmaking practices and chemical and mechanical properties of steel besides consideration of forging and post forging processes which the end product has to undergo. Selection of steels for forging also depends on the forming methods, machining operations, heat treating procedures, and deterioration of properties with time in service, as well as the conventional mechanical and chemical properties of the steel to be forged.

An efficient forging design obtains maximum performance from the minimum amount of material consistent with the loads to be applied, producibility, and desired life expectancy. To match the steel to its design component, the steel is first appraised for strength and toughness and then qualified for stability to temperature and environment. Optimum steels are then analyzed for producibility and finally for economy.

The types of steels which are used for the forging operations, range in composition from around 0.1 % to 1 % carbon and from trace amounts to around 5 % each of several metallic alloying elements (such as chromium, molybdenum, vanadium, and nickel etc.). The carbon content controls the hardness and strength levels obtainable, while the alloying  elements serves the purposes of increasing hardenability and hot strength, improving resistance to thermal and mechanical shock, providing grain refinement, and improving the machinability. The steels in this group range from standard carbon and low-alloy steels to specially alloyed, super strength steels. At their respective forging temperatures, all of these steels show the same basic forging behaviours.

Forging quality steels are those steels which are subjected to the process of forging during its subsequent processing for the production of end use products. The process of forging consists of converting the steel material into designed shape at a high strain rate. The extent to which carbon and low-alloy steels can be forged into intricate shapes is seldom limited by the forgeability problems, except when the grades contain sulphides, bismuth, or the intentional additions for other purposes (for example free machining). Probably the most important factor limiting the section thickness, shape complexity, and forging size is the cooling which occurs when the heated work-piece comes in contact with the colder dies. This is essentially the reason that more intricate shapes can be forged in forge hammers than in forge presses, under otherwise similar conditions of stock and die temperatures.

Steels of forging quality are produced to a wide range of chemical compositions by electric furnace, or basic oxygen furnace steelmaking processes. With each of the melting and rolling practices, a level of testing and evaluation of quality is to be exercised. The details of testing and quality evaluation can vary from producer to producer. However, if the designer needs it, one or more of special quality restrictions can be specified. These brings into effect additional qualification testing by the producing plant. Electric arc furnaces have the ability to produce forging-quality steel by a single slag melting practice. All forging quality steels are to be fully killed. Further, forging steels are required to have several metallurgical properties such as (i) good surface quality, (ii) good cleanliness, (iii) good internal quality, (iv) required impact toughness, (v) fine and uniform grain size of austenitic grains, and (vi) good response to heat treatment.

High strength alloy steels, because of their low tolerance to inclusions, need high level of refinement to reduce the occurrence of non-metallic inclusions. Occasionally, for higher-reliability applications, it is necessary to treat the liquid steel by vacuum arc remelting or electroslag remelting processes. Steels produced to these quality levels are subject to highly restrictive evaluation procedures. A sufficient amount of material is to be cropped from the top and bottom ends of ingots / cast products to ensure that the forgings are free from harmful segregations. This term includes all in-homogeneities which are liable to impair the required characteristics.

Raw material used for forging is normally in the form of cast ingots, blooms or slabs, or rolled blooms, slabs, billets, squares, flats, rounds, an wire rods. Im case of the rolled steel, the material has normally received considerable reduction before the forging operations. For certain grades, vacuum melting imparts better forgeability. However, the major purpose of vacuum melting is for the improvement of the mechanical properties, and not the forging behaviour.

Forging quality steels have the property of forgeability which is the relative ability of the steel to flow under compressive loading without fracturing. Except for resulphurized and rephosphorized grades, majority of carbon and low-alloy steels are normally considered to have good forgeability. Differences in forging behaviour among the various grades of steel are small enough and the selection of the steel is seldom affected by forging behaviour. However, the choice of resulphurized or rephosphorized steel for a forge product is normally justified only if the forge product is to be extensively machined, since one of the principal reasons for considering manufacture by forging is the avoidance of subsequent machining operations. However, this situation is quite uncommon.

Forgeability evaluations at a particular temperature do not necessarily define the ease with which a metal or an alloy can be forged in dies under shop conditions. In die forging operations, metal / alloy temperatures normally vary because of die chilling and because of energy absorption, which causes heating. As a result, a metal with a wide forging temperature range can be easier to forge than the metal which withstands equal amounts of deformation without rupture in standardized tests.

Forgeability evaluation of metal and alloys is normally being carried out mainly by the hot twist test. This test is a common means of measuring the forgeability of steels. As the name implies, this test involves twisting of heated bar samples to fracture at a number of different temperatures selected to cover the possible hot working temperature range of the test material. The number of twists to fracture, as well as the torque needed for maintaining of twisting at a constant rate, is reported. The temperature, at which the number of twists is the greatest, if such a maximum exists, is assumed to be the optimal hot working temperature of the test material. Fig 2 shows forgeabilities of several carbon steels as determined by the hot-twist test.

Fig 2 Forgeabilities of carbon steels as determined by hot-twist test

There are some other tests for determining the forgeability of steels. In the wedge-forging test, a wedge-shape sample is forged between flat dies and the vertical deformation which causes cracking is established. The side-pressing test consists of compressing a cylindrical bar sample between flat, parallel dies with the axis of the cylinder parallel to the dies. The ends of the cylinder are unconstrained, and forgeability is measured by the amount of deformation obtained before cracking. In the upset test, a cylinder is compressed between flat dies and the surface strains at fracture at the equator of the cylinder are measured. The notched-bar upset test is similar to the upset test except that axial notches are machined into the test sample to introduce high local stress levels. These higher stresses can be more indicative of the stresses experienced during actual forging operations than those produced in the standard upset test. · The hot tensile test frequently uses a special test apparatus to vary both strain rates and temperatures over a wide range

The proper choice among forgeability tests depends somewhat on the properties of the metal / alloy being tested. Based on known correlations of test values with forging performance, the hot-twist test is particularly useful for evaluating the forgeability of carbon, low-alloy, alloy, and stainless steels, which are forged relatively high in their hot-working temperature ranges. However, the test fails to correlate with the forging performance of metals / alloys deformed at comparatively low temperatures since the work-hardening effects cause the twisting zone to shift during testing. Tension tests, performed either in impact or tensile-test machines, provide data on ductility which agree to some extent with the forgeability of metals / alloys at cold-working temperatures.

The hot forging of carbon and alloy steels into intricate shapes is rarely limited by forgeability aspects with the exception of the free-machining grades mentioned earlier. Section thickness, shape complexity, and forging size are limited primarily by the cooling which occurs when the heated work piece comes into contact with the cold dies. For this reason equipment which has relatively short die contact times, such as forge hammers, is frequently preferred for forging intricate shapes in the steels.

Carbon steel grades are the most common steels used for forging applications. Low carbon steels (carbon between 0.1 % and 0.25 %) are the easiest to cold forge due to their soft and ductile nature. Medium carbon steels (carbon between 0.26 % and 0.59 %) are typically used in medium and large parts forgings. High carbon steels (carbon above 0.6 %) are used for applications in which high strength, hardness, and wear resistance are necessary, such as wear parts, gear wheels, chains, and brackets.

Quenched and tempered steel grades are hardenable steels. They are alloyed with chromium and molybdenum for example, to favour transformation of austenite into martensite during the quenching process. The forging part is quenched in water, polymer, or oil to obtain the required hardness. The tempering process enables the mechanical properties and toughness to be adjusted.

Case hardening steels are used for parts which need high surface wear resistance while retaining a soft core that absorbs stresses without cracking. After forging and machining, the outer layer is carburized and / or carbo-nitrided and then locally hardened by quenching. The grades are normally low-carbon steels to which suitable alloying elements have been added. A special characteristic of this kind of grade is the Jominy curve, which needs to be well controlled.

The use of micro alloyed high strength low alloy (HSLA) steels has evolved as an alternative to iron castings for applications such as automotive crankshafts etc. These steels typically have small additions of vanadium, niobium or titanium (0.05 % to 0.10%), which increases yield strength by precipitation hardening, while offering finer grain structures. These two outcomes increase the strength of the forged parts compared to conventional carbon steels. Micro-alloyed steel grades allow the production of parts with higher strength, obtained without subsequent heat-treatment after forging. As a result, these steels retain the advantages of the forging process while being economically competitive with castings because of the elimination of the heat-treat cycle.

Bainitic steels are designed for applications which need both high properties and process cost reduction compared with quenched and tempered grades. Very high strength can be achieved (ultimate tensile strength greater than 1,100 MPa) without heat treatment. Controlled cooling after forging steers the austenite transformation into the bainitic region. The desired level of strength is reached by fine-tuning the alloying elements taking into account the subsequent customer’s processes and the size of the part. Bainitic steels achieve higher mechanical properties than micro-alloyed grades as well as demonstrate uniform hardness throughout the steel.

At the more demanding end of the spectrum for steel forgings is the series of precipitation-hardenable stainless steels. These steels provide an excellent combination of high strength, toughness, and corrosion resistance. These steels are normally produced as vacuum induction melted plus consumable electrode remelted products. The hardening mechanism includes solution heat treating and cooling at a rate sufficient to retain solute atoms or compounds in the supersaturated state. The forgings are normally supplied in this condition to facilitate subsequent machining operations. At the final stages of fabrication and assembly, the material is then aged or precipitation hardened to develop its desired strength, toughness, and corrosion resistance. Forgings produced from this family of steels are typically utilized in the most demanding applications, such as aerospace and marine environments.

Stainless steels are widely used where resistance to heat and corrosion is required, in applications upto around 510 deg C. The main feature is the chromium content of at least 10.5 %. Corrosion resistance is ensured by a tight chromium oxide layer on the steel surface. Further alloying elements are added to adapt the steel in line with specific additional requirements such as corrosion resistance against sea water, high hardness and tensile strength etc. The popular stainless steels used for forging include grades 304, 304L, 316, and 316L. Stainless steels are higher in cost than carbon, micro-alloy and alloy steels. They are hot forgeable into simple shapes and low profile structural shapes, but the high forging pressures restricts net shape forging to simpler shapes. The hot die forging process is required to be reviewed when more complex shapes are encountered or when the more difficult to forge alloys are specified. 300 series stainless steels need 20 % to 40 % higher forging pressures than needed for the 400 series of stainless steels, mainly due to the higher nickel content.

Carbon and alloy steels are by far the most commonly forged materials, and are readily forged into a wide variety of shapes using hot forging, warm forging, or cold-forging processes and standard equipment. Despite the large number of available compositions, all of the materials in this category show essentially similar forging characteristics. Exceptions to this are steels containing free-machining additives such as sulphides. These materials are more difficult to forge than are non free machining grades.

Normally, the hot forgeability of carbon and alloy steels improves as deformation rate increases. The improvement in the workability has been primarily attributed to the increased heat of deformation generated at high deformation rates. Selection of forging temperatures for carbon and alloy steels is based on carbon content, alloy composition, the temperature range for optimum plasticity, and the amount of reduction required for the forging of the work piece. Of these factors, carbon content has the most influence on upper-limit forging temperatures.

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