Hadfield Manganese Steel
Hadfield Manganese Steel
Hadfield steel also known as Hadfield steel or Hadfield austenitic manganese steel was invented by Sir Robert Hadfield in 1882. The first British patent for this steel, number 200 was granted for this steel in 1883. The first US patents, numbers 303150 and 303151, were granted in 1884. Fig 1 shows the commemorative plaque of Sir Robert Hadfield cast in the Hadfield manganese steel.
Hadfield had done a series of test with the addition of ferro-manganese containing 80 % manganese and 7 % carbon to de-carbonized iron. Increasing manganese and carbon contents led to increasing brittleness upto 7.5 % manganese. At manganese contents above 10 %, however, the steel became remarkably tough. The toughness increased by heating the steel to 1,000 deg C followed by water quenching, a treatment which would render carbon steel very brittle. This alloy steel which was introduced commercially contained 1.2 % carbon and 12 % manganese in a 1 to 10 ratio.
Hadfield manganese steel is the basic target for much of the manganese steels produced even to this day. The chemistry has not changed much since the discovery of this alloy steel. Majority of the producers do offer similar materials with some variations to the carbon level, with or without other added elements, but these alloy steels are still roughly 12 % manganese. By varying the carbon level, the desired properties can be optimized.
Hadfield manganese steel is unique in that it shows resistance to impact, high toughness, high ductility, high work hardening ability, excellent wear resistance, and slow crack propagation rates, in comparison to other potentially competitive materials. It is also non-magnetic and can work harden during service or can be surface-hardened to as high as 500 HB by mechanical or explosive means prior to service. Because of these properties, Hadfield manganese steel gained rapid acceptance as a useful engineering material.
Hadfield manganese steel has a proven high resistance to abrasive wear including blows and metal-to-metal wear, even though it has a low initial hardness. This steel is supposed to work harden under use and thus gives a hard-wear resistant surface and has a good wear resistance in components even without heavy mechanical deformation.
Hadfield manganese steel has the ability to harden in-depth in service as well as by induced means. Work hardening is usually induced by impact, as from hammer blows. Light blows, even if they are of high velocity, cause shallow deformation with only superficial hardening even though the resulting surface hardness is ordinarily high. Heavy impact produces deeper hardening which is normally with lower values of surface hardness. Hadfield manganese steel is unequalled in its ability to harden, exceeding even the metal stable austenitic stainless steel.
The hardness of Hadfield manganese steel in the solution annealed and water quenched condition is normally around 200 HB. It is possible to strain harden this material to around 500 HB. In order to achieve this high hardness level, the impact loading is to be high while the material wearing away from gouging abrasion is limited. It is typical in crushing applications where the main wear mechanism is gouging abrasion that the Hadfield manganese steel hardens to some intermediate level, typically 350 HB to 450 HB.
The mechanisms by which hardening occurs include twinning, stacking fault formation and dynamic strain. Fig 1 shows a work-hardened microstructure of manganese steel in which clearly visible slip lines are the result of the work hardening to the grains. It has also been suggested that a deformation based martensitic transformation also plays a part in the hardening, but no evidence has been found to support this theory.
Fig 1 Hadfield manganese steel
he rate of work hardening of the Hadfield manganese steel is dependent primarily on two factors. The first factor is the rate and intensity of impacts being received by the material. This is a not a property of the material, but a result of the service conditions. More impacts and those of larger magnitude speed the material along to its maximum hardness level. The second factor is the amount of carbon in the material. The higher the carbon level the faster the material work hardens in any given service.
In order to increase dimensional stability or add additional wear life to the Hadfield manganese steel parts, the parts can also be pre-hardened by explosive hardening. In this process, the surface to be hardened are covered with sheet explosives and the pressure created by the explosion deforms and hardens the surface. Sometimes this process is repeated two or three times to achieve a higher hardness to a greater depth. Railroad track work castings are an example of Hadfield manganese steel castings which are frequently explosively hardened.
Hadfield manganese steel is still extensively used in the several heavy industries, in the form of castings or rolled shapes. The heavy industries using Hadfield manganese steel are earthmoving, mining, quarrying, oil and gas drilling, steelmaking, dredging, logging and lumber, and railways. In the railway industry, Hadfield manganese steel has been used successfully in special track work. Due to its austenitic structure, Hadfield manganese steel has no magnetic response and hence it makes good wear plates for the bottom of electro-magnets.
The ability of Hadfield manganese steel to work harden from impact loading along with its exceptional toughness make it the best wear material choice for many demanding applications. These applications include (i) gyratory crusher mantles, (ii) concaves, jaw crusher dies, (iii) cone crusher bowl liners and mantles, (iv) track pads for large mining shovels, and (v) hammers for many different types of impact crushers including automobile shredders. Hadfield manganese steel also develops a favourable wear pair with alloy steels so Hadfield manganese steel can be used as a bushing material in demanding mining applications. Frogs, diamond crossings, guardrail wear bars and replaceable switch point tips for the railways are also produced from Hadfield manganese steel. Various conveyor links, flights, buckets and pans are also made from this steel, Armour plate and even soldiers helmets have been produced from Hadfield manganese steel in the past. Fig 2 shows examples of application of Hadfield manganese steel.
Fig 2 Examples of the application of Hadfield manganese steel
Hadfield manganese steel also finds applications at extremely low temperatures due to its persistent toughness even at these temperatures. One application of this steel which is not a wear component is in the construction of safes since this steel resists many of the normal break-in methods used by the miscreants so the safes made from this steel are quite robust. Hadfield manganese steel also works well in most situations which involve impact or gouging abrasion where other alloy steels fail.
Production of Hadfield manganese steel
Hadfield manganese steels can be produced by any of the conventional steelmaking processes. In the early days of its production, molten ferro-manganese was added to essentially carbon free iron to produce this steel. This process required two different melting furnaces and was known as the duplex process. Present day production of Hadfield manganese steel is carried out in electric arc furnaces or electric induction furnaces. Due to the attack of manganese oxides on acid type furnace linings, a basic or neutral lining is to be used in the induction melting furnace to produce this steel.
The furnace charge is typically made up of foundry returns (gates and risers), worn castings, ferro-alloys and steel scrap. Many producers dead melt the charge to produce the Hadfield manganese steel, but it is also possible to perform a silicon burn in the electric arc furnace. The silicon burn helps to stir the metal, remove oxides and lower gas levels. Arc furnace heats normally employ a single slag practice to protect the metal during melting. This slag is produced with lime (CaO) and is to be kept basic in nature to prevent excessive attack of the basic furnace lining.
Hadfield manganese steel is quite frequently used as cast steel in the industry. Sand and metal moulds are used for high manganese steel castings. The fluidity of the Hadfield manganese steel is quite high, approaching that of the cast iron, which makes it is possible to fill intricate shapes and pour at low superheats. The structure of the as-cast Hadfield steel consists of austenite and excess carbides, which are responsible for enhanced ductility and wear resistance. In almost every case, the steel is subjected to quenching. The Hadfield manganese steel castings are only rarely subjected to machining as these are not easily workable due to the hardening effect which can occur during cutting and the resultant accelerated wear of the tool.
In order to achieve a fine-grained material it is necessary to use low superheats when pouring Hadfield manganese steel. Grain refiners for manganese steels do exist, some of which are proprietary to different producers and others of which are commercially available. Some producers have had some success implementing the various grain refinement techniques, but the most reliable method to obtain a fine grain structure is to pour the metal with low superheat.
Figure 3 show the fracture surfaces of 50 mm by 50 mm square bars which were broken as cast to reveal the grain size. Both bars are from the same (12 % manganese) heat with bar at Fig 3a being poured at 1380 deg C and bar at Fig 3b being poured at 1485 deg C. In practice, pouring temperatures below 1425 deg C are typical for the 12 % manganese grades and can be much lower as carbon levels are increased. Temperatures near 1370 deg C are desirable for pouring the higher manganese and carbon grades.
Fig 3 Structure of cast Hadfield manganese steel
In order to successfully pour at these low superheats, it is important to have a well insulated and preheated ladle. Ladles are typically lined with a basic or neutral (alumina based) refractory lining to avoid attack from the manganese oxides. It is also normal to pour at a fairly high volumetric rate to prevent laps and wrinkles. Care is required to be taken to avoid excessive air entrainment due to the turbulence when pouring at low superheats. When these gases are entrained, they are likely to be trapped at the top surface of the casting. This can lead to costly weld repairs or scrapped castings.
Moulds for Hadfield manganese steel castings, just like the furnace and ladle, are subject to reaction with the manganese oxides. In order to avoid this problem, many producers use olivine sand to make the moulds. This sand is basic in nature and does not react with manganese oxides. It is possible to use silica sand for the moulds, but the mould either requires to be faced with olivine or mould heavily washed with an appropriate coating. Even properly coated silica sand moulds can produce poor quality surface finishes. Any of the standard sand bonding systems works with Hadfield manganese steels, but not all the systems work with olivine sand. Other details of the mould are similar between Hadfield manganese steel and carbon or alloy steels.
Hadfield manganese steel is a wide freezing range steel which leads to dispersed micro-shrinkage and micro-porosity. This makes it difficult to feed to high levels of solidity and thus allows for the use of smaller risers than an equivalent casting poured in carbon steel. Yields can reach higher than 65 % in some cases. The pouring temperature also affects the required amount of risering. Low pouring temperatures allow for minimal risering. This methodology produces adequate internal soundness for most applications except armour plate. When Hadfield manganese steel is to be used for armour plate, it is essential to heavily riser the casting. This produces the soundest casting and a high quality armour plate.
Heat treatment – Ideally heat treated Hadfield manganese steel has a fully homogenized fine-grained austenitic micro-structure. The grain size is a function of pouring temperature and heat treatment typically does not influence the grain size. Some producers have tried to develop strategies of heat treatment which first transforms the structure to a pearlitic structure, which then allows for grain refinement in the final heat treatment. These strategies have not been widely accepted or implemented for different reasons. One reason is that these cycles become expensive due to the high furnace temperatures and long hold time periods needed. In addition the steel is frequently not significantly improved by these cycles.
The typical heat treatment cycle for the Hadfield manganese steels consists of a solution anneal followed by a water quench. This cycle can start off at room temperature or at an elevated temperature depending on the starting temperature of the castings. The starting temperature in the heat treatment furnace is set to be near the castings temperature and is then raised at a slow to moderate rate until the soaking temperature is reached. Soaking temperatures are typically high in order to facilitate the dissolution of any carbide which can be present. Temperatures at or near 1,100 deg C are typically used to achieve the desired homogenizing effect. The chemical composition of the steel ultimately sets the soaking temperature.
Hadfield manganese steel castings need a rapid water quench following the high temperature soak. This quench needs to occur immediately after the castings are removed from the heat treatment furnace. The rate of this quench needs to be high enough to prevent any precipitation of carbides. Fig 3 shows the microstructure of properly quenched Hadfield manganese steel. A slack quench can reduce the toughness of the material dramatically. In the toughened condition manganese steel castings can be final processed with little special care.
The one item to avoid with heat-treated Hadfield manganese steel castings is reheating above 260 deg C. Temperatures at or above this level causes the precipitation of acicular carbides, which can dramatically reduce the toughness. This effect is time and temperature based with longer times and higher temperatures both causing greater losses of toughness.
Cleaning – The removal of gates, risers and vents can be carried out in a few different ways for the Hadfield manganese steel castings. If the castings are allowed to cool to room temperature after shakeout, most of the rigging can be broken or flogged off of the casting. The casting is fairly brittle at this point and removal of these items with an impact force can be quite effective. Once heat-treated, however, it is no longer be possible to break anything or size off of the casting.
Cutting is needed to remove rigging from the casting once it has been heat-treated. This can be done by abrasive cutting, torch cutting, or arc air gouging. Torch cutting is somewhat difficult and produces high volumes of smoke, due to the high alloy content of Hadfield manganese steel. It is typical to use torch tip sizes which are much larger than what is needed for carbon steel.
Properly designed dust and smoke collection equipments are needed when using hot methods to cut Hadfield manganese steel. Care is also to be taken not to overheat the Hadfield manganese steel when doing hot work. Fast cuts and moving around the casting to avoid concentrating the heat are advisable in order to minimize the damaging effect of overheating.
Welding – Hadfield manganese steel is weldable. Just like for any other hot work on Hadfield manganese steel, the welding inter-pass temperature is to be kept below 260 deg C to avoid embrittlement. No post weld stress reliefs are needed or desirable for the Hadfield manganese steel. Heavy peening of each weld bead is desired to set up compressive stresses in the weld.
Before welding begins, proper surface preparation is needed. It is typical to have a decarburized surface on castings which is to be removed before welding. Worn Hadfield manganese steel parts have a work hardened layer which also is to be removed to facilitate welding. For joining or fabrication it is typical to use a stainless steel welding consumable. This material adheres well to the Hadfield manganese steels.
Weld filler materials which are also manganese steels are available and are frequently used for repair welding of castings. Few, if any, of these materials are to be able to match the base metal wear resistance. High carbon contents are critical to allow Hadfield manganese steel to resist the wear which it typically encounters in service. Carbon levels are somewhat limited in weld filler materials so that the produced welds do not crack after being deposited.
Machining – Hadfield manganese steel is well known difficult-to machine material due its chemical composition containing high manganese. The machining of Hadfield manganese steel has various problems due to the material’s work-hardening and low thermal conductivity properties as well as high hardness. These properties have an important effect on machinability of the material and hence, the selection of optimum machining parameters is vital to achieve higher material removal rate, longer tool life, and better surface finish.
The unique wear resistant properties of Hadfield manganese steel also make it very difficult to machine. In the early days of Hadfield manganese steel production, it was thought to be un-machinable and grinding was used to shape the parts. Now with modern cutting tools, it is possible to turn, bore and mill Hadfield manganese steel. This steel does not machine like other steels and typically needs tools which are made with a negative rake angle. In addition, relatively low surface speeds with large depths of cut produce the best results. This arrangement produces high cutting forces and the equipment and tooling is to be robust to withstand these forces. Any chatter of the tooling can add to the work hardening of the surface being machined.
Most cutting is typically done without any sort of lubrication. During the machining of Hadfield manganese steel, it is important to continuously remove the work-hardened zone with the next cut. Small finishing cuts or tool chatter causes the hardness to build and make the remaining surface virtually un-machinable.
Drilling of Hadfield manganese steels, while possible, is very difficult and requires holes to be cast into the part. If drilled holes are needed, mild steel inserts are frequently cast into the part so that the machinable insert can be drilled or drilled and tapped.
Properties of Hadfield manganese steel
Physical properties – The physical property of the Hadfield manganese steel in the properly heat treated condition are (i) melting point – liquidus 1400 deg C, and solidus – 1350 deg C, (ii) specific gravity – 7.87 at 15 deg C, and (iii) density – 7.89 grams per cubic centimeters.
Chemical composition – Essentially, Hadfield manganese steel is a solid solution of carbon and manganese in gamma iron. Fig 4 indicates a phase diagram for steel containing 13 % manganese. While the nominal composition of Hadfield manganese steel is 1.2 % carbon and 12 % to 13 % manganese, in the commercial products of Hadfield manganese steel, the carbon ranges between 1 % and 1.4 %, while the manganese content is in the range of 10 % to 14 % as per the specification ASTM–A128.
Fig 4 Phase diagram and gouging wear ratio
Carbon is one of the two most important elements in Hadfield manganese steel along with manganese. Hadfield manganese steel is a supersaturated solution of carbon. For the standard grade of Hadfield manganese steel, the carbon and manganese are in an approximate ratio of 10. Hence, this steel typically has 12 % of manganese and 1.2 % of carbon. This ratio was mainly setup by early steel making limitations and the fixed ratio has no real significance. Increasing the carbon content raises the yield strength and lowers the ductility.
The main significance of increased carbon content though is to increase the gouging wear resistance. Since the Hadfield manganese steel is used in gouging abrasion and high impact wear situations so the producer of this steel try to maximize carbon content. Practical limits do exist and as the carbon content exceeds 1.3 %, cracking and undissolved grain boundary carbides become more prevalent. However, some premium grades of Hadfield manganese steels which are with high manganese content, have the upper carbon limit well beyond 1.3 %.
Carbon content affects the yield strength of Hadfield manganese steel. Carbon levels below 1 % cause yield strengths to decrease. The optimum carbon content has been found to be between 1 % and 1.2 %. Above 1.2 % carbon content, yield strength is unaffected. Other alloying elements such as chromium increase the yield strength, but decrease ductility. Silicon is generally added as a de-oxidizer. Carbon contents above 1.4 % are not generally used as the carbon segregates to the grain boundaries as carbides and is detrimental to both strength and ductility.
Manganese being an austenite stabilizer makes the family of Hadfield manganese steels possible. It decreases the austenite to ferrite transformation temperature and hence helps to retain a fully austenitic structure at room temperature. Hadfield manganese steel with 13 % manganese and 1.1 % carbon has a martensite start temperature below minus 200 deg C. The lower limit for manganese content in plain austenitic manganese steel is near 10 %. Increasing manganese levels tend to increase the solubility of nitrogen and hydrogen in the steel. Premium grades of Hadfield manganese steel with higher carbon contents and additional alloying elements exist with manganese levels in the range of 16 % to 25 %. These grades are proprietary to their producers.
Manganese has very little effect on the yield strength of Hadfield manganese steel, but does affect both the ultimate tensile strength and ductility. Maximum tensile strengths are attained with 12 % to 13 % of manganese content. Although acceptable, mechanical properties can be achieved upto 20 % manganese content, there is no economical advantage in using manganese content higher than 13 %. Manganese acts as an austenitic stabilizer in Hadfield manganese steel and delays isothermal transformation. For example, carbon steel containing 1 % manganese begins isothermal transformation about 15 seconds after quenching to 370 deg C, whereas steel containing 12 % manganese begins isothermal transformation around 48 hours after quenching to 370 deg C.
The structure of Hadfield manganese steel is essentially one of carbon in solution in austenite. The practical limit of carbon in solution is around 1.2 %. Thereafter, excess carbon precipitation to the grain boundaries results, especially in heavier sections.
Mechanical properties – The typical mechanical properties of Hadfield manganese steel for different plate thickness are given in Tab 1. Although stronger than low carbon steel, it is not as strong as medium carbon steel. It is however, much tougher than medium carbon steel. Yielding in Hadfield steel signifies the onset of work hardening and accompanying plastic deformation.
|Tab 1 Typical mechanical properties of Hadfield manganese steel|
|Sl. No.||Plate thickness||Grain size||Tensile strength||Elongation||Reduction of area||Impact value (Izod)|
|Composition – Carbon 1.1 %, Manganese 12.7 %, Silicon 0.5 %, Phosphorus 0.043 %|
The modulus of elasticity for Hadfield manganese steel is 186,000 MPa and is somewhat below that of carbon steel, which is normally taken as 200,000 MPa. The fatigue limit for Hadfield manganese steel is around 270 MPa.
However, the mechanical properties of Hadfield manganese steel vary significantly with the section size. Properties affected are tensile strength, elongation, reduction of area and impact resistance. For example, a 25 mm thick section properly heat treated, displays higher mechanical properties than 100 mm thick section. Grain size is the primary reason for these differences in the mechanical properties. Fine-grained Hadfield manganese steel samples show tensile strengths and elongations upto 30 % higher than course grained Hadfield manganese steel samples.
The ability of Hadfield manganese steel to work harden upto its ultimate tensile strength is its main feature. In this regard Hadfield manganese steel has no equal. The range of work hardening of Hadfield manganese steel from yield to ultimate tensile is around 200 %. This however is accompanied by large dimensional instability. Fig 5 gives a typical stress strain curve for Hadfield manganese steel. Fig 5 also gives two tensile samples out of which one is a Hadfield manganese steel bar shown on the top and the other an alloy steel bar shown on the bottom. The alloy steel bar has necked and failed with the normal cup-cone fracture. The Hadfield manganese steel bar on the other hand necked along its entire length. Each time the bar starts to yield; the work hardening of that area increases the strength and prevents further necking until the whole length has necked. This progression is repeated until failure.
Fig 5 Tensile testing of Hadfield manganese steel
Hadfield manganese steel is capable of supporting a compressive load of 4,825 MPa without rupture. The fatigue limit of the wrought material is 415 MPa, based on 10 million cycle reversing stress. Due to its high level of toughness, Hadfield manganese steel is resistant to overstress crack propagation, but cyclically loaded parts like crusher liners can succumb to fatigue if not properly supported.
Thermal properties of Hadfield manganese steel – The typical thermal properties of Hadfield manganese steel are given in Tab 2.
|Tab 2 Typical thermal properties of Hadfield manganese steel|
|Mean specific heat|
|Temperature range in deg C||50-100||150-200||350-440||550-600||750-800||950-1,000|
|Value in cal/g.deg C||0.124||0.135||0.145||0.168||0.155||0.161|
|Mean thermal expansion co-efficient|
|Temperature range in deg C||0-100||0-200||0-400||0-600||0-800||0-1,000|
|0.000001 cm/cm deg C||18||19.4||21.7||19.9||21.9||23.1|
|Instantaneous thermal expansion coefficient|
|0.000001 cm/cm deg C||18.4||19.1||20||21.4||22.7||24.1||24.5|
|The thermal expansion of Hadfield manganese steel is 1.5 times to 2 times that of carbon steel|
|The thermal conductivity of steel is around 3 times higher than that of Hadfield manganese steel|
As cast properties – Hadfield manganese steel in the as-cast condition is too brittle for normal usage. As section thickness increases, the cooling rate within the moulds decreases. This decreased cooling rate results in increased embrittlement due to carbon precipitation. In as castings, the tensile strength ranges from around 345 MPa to 485 MPa and displays elongation values below 1 %. Heat treatment is used to strengthen and increase the mechanical properties of Hadfield manganese steel. The normal heat treatment method consists of solution annealing and rapid quenching in a water bath.
Solution annealing normally consists of re-heating the steel to a high enough austenitizing temperature above 1,010 deg C for a sufficient time period of time to complete the solution of carbon. Normally railway frogs and crossings are heat treated at 1,095 deg C and held for 2 hours for each 25 mm of thickness before quenching.
Quenching is accomplished by immersion in a water bath agitated by air, since agitation reduces the tendency for the formation of a vapour coating (known as the Leidenfrost effect) on the casting surfaces, and hence a more uniform rate of cooling is achieved. The speed of quench is an important factor in the final mechanical properties. The maximum rate of quenching is fixed by the heat absorption from the casting surface by agitated water and by the rate of thermal conductivity of the Hadfield manganese steel. As given in Tab 2, Hadfield manganese steel has low thermal conductivity.
A lower rate of quench, results in lower mechanical properties in the centre of heavier section. This results in a practical maximum thickness for castings to around 150 mm. Another reason for limiting casting size to 150 mm is that castings larger than 150 mm develop large residual stresses upon cooling in the mould. Such stresses acting on a brittle steel structure are prone to cause cracking in heavier sections prior to heat treatment.
Heat treatment at high temperature causes surface decarburization and some loss of manganese. This decarburization layer can be as much as 3 mm and can be slightly magnetic. This is not normally a problem, as most railway frog and crossing running surfaces are machined by either milling or grinding, which removes this decarburization layer.
Hadfield manganese steel has an as cast as heat-treated hardness of around 190 HB to 200 HB. It also possesses relatively low yield strength. As hardness is a measure of resistance to plastic flow (yielding), work hardening of the cast wear surface of railway castings for frogs and crossings is needed. This hardness increase in turn increases the yield strength and results in a surface more resistant to impact and flow. The current normal accepted practice is to explosive depth harden railway castings for heavy haul use. This process results in surface hardness of 370 HB to 385 HB.