ABRASION WEAR RESISTANT STEEL

Abstract: Austenitic manganese steel characterised by the addition of sufficient carbide forming elements to promote the retention of austenite during cooling from solution treatment, produce a dispersion of undissolved primary carbides in the austenite matrix and retard decomposition of the retained austenite when the steel is reheated. Steels containing, by weight percent, 0.6-2.5 C, 10-17 Mn and 4-10 Cr (possibly with Ni), Mo or Ti are exemplified.

Description: "Abrasion Wear Resistant Steel" THIS INVENTION relates to a range of improved austenitic manganese steel which can be selected for improved weld- ability, castability, wear resistance, high yield streng¬ th, high temperature resistance, low temperature impact resistance and/or corrosion resistance. PRIOR ART Conventional austenitic manganese steel is used exten¬ sively in heavy duty applications where resistance to gouging and abrasion wear is required. Typical applica¬ tions of such steel includes, jaw crusher liner plates, gyratory cone crusher mantle and bowl liners, grizzly bars and hammer mill impactors. The usual chemical composition of such steel is:- Carbon 1.0 - 1.4% Manganese 10 - 14% Iron balance (All compositions given throughout the specificiation and claims are by weight per cent). There are of course some minor variations which might in¬ clude chromium or molybdenum of up to approximately 2% and other elements in trace amounts. After casting, the steel is solution treated at 1050°C followed by water quenching to produce a tough, relatively soft (200 HV appro . ) steel with a characteristic capacity to work harden at and near the surface during service to a hardness in the order of 550 HV. Thus, conventional austenitic manganese steel has a tough interior with a hard, abrasion wear resistant surface which continuously forms in service as the outer layer are worn away by an abrasive medium. Such conventional austenitic manganese steels have a num¬ ber of inherent characteristics which impose severe limi¬ tations upon their application. These characteristics include:- a. The final treatment necessary to develop the charac¬ teristic toughness and high work hardening capacity of conventional austenitic manganese steel consists of rapid cooling from the solution threatment tem¬ perature. The rate of quenching is important since it is necessary to exceed a minimum critical cooling velocity to prevent the formation of embrittling carbides. This cooling rate requirement imposes a limitation on the maximum size of the steel section that can be effectively toughened. b. Conventional austenitic manganese steel exhibits a relatively low yield stength in the solution treated condition and readily undergoes plastic deformation in service prior to work hardening. Consequent undesirable loss in dimensional tolerances occurs. c. Suitably toughened conventional austenitic manganese steel readily decomposes during re-heating (eg. by conventional welding operations) due to the formation of embrittling carbides resulting in a loss of tough ness and decrease in abrasion wear resistance. The austenitic structure in conventional manganese steel is metastable and decomposes at temperatures above 350°C to form a number of embrittling phases as shown in the graph set forth in Fig. 1 of the accompanying drawings which is taken from Collette, G Crussard, C Kohn, A Plateau, J Pomey, G. et. Weisz, M., Rev de Met., (1957), 54, 433. OϊH The influence of the presence of the embrittling phases on the mechanical properties of conventional austenitic man¬ ganese steel is shown the graph set forth in Fig. 2 of the accompanying drawings which applies to a conventional austenitic manganese steel having a content of 13% manga¬ nese and 1.2% carbon (taken from Imai, Y. and Saito, T.: Sci. Rep. RITU, (1962) A14, 92). Therefore in summary, the application of heat to conven¬ tional austenitic manganese steel severely embrittles the steel which may result in catastrophic failure when in service. In addition, there is a considerable reduction in the abrasion wear resistance of the steel. British Patent Specification No. 2007257B discloses an austenitic manganese steel for railway frogs containing 0.75 to 0.9% carbon, not more than 1% silicon, 12.5 to 15% manganese and 1.0 to 1.8%, molybdenum and which is weld- able. This composition is very close to the ASTM A128 Grade E-l specification for austenitic manganese steel for castings. Each of these products is essentially a lower carbon alloy than conventional manganese steel and is generally known to be less suσceptible to embrittlement on reheating, such as occurs during welding. However con¬ trary to the test results given in the aforementioned patent specification manganese steel containing 0.8% car¬ bon has a lower work hardening capacity (i.e. wear resis¬ tance) than conventional manganese steel containing 1.2% carbon (White D.H. and Honeycombe R.W. ., "Structural changes during the Deformation of high purity iron-manga¬ nese-carbon alloys" - Journal of the Iron and Steel Insti¬ tute, Vol. 200, page 457, (1962). ' THE INVENTION As a result of research, it has been determined that the austenitic structure is retained during cooling from solution treatment and the formation of the embrittling phases referred to in Fig. 1 in austenitic manganese steel may be substantially retarded during heating above 350°C by the addition of sufficient quantities of one or more carbide forming elements, such as chromium, molybdenum, titanium, niobium, tungsten; and vanadium. Although it can be said that the molybdenum referred to in the aforementioned British patent is a carbide forming element the amount used is not sufficient to significantly retard the decomposition of the austenitic structure and does not overcome the severe limitations of conventional austenitic manganese steels discussed above. The present invention is based on the realisation that by using appr- priate amounts of carbide forming elements these limita¬ tions are substantially removed. Thus in one form the invention resides in an austenitic manganese steel characterised by the addition of suffi¬ cient carbide forming elements to promote the retention of austenite during cooling from solution treatment. In another form the invention resides in an austenitic manganese steel characterised by the addition of suffi¬ cient carbide forming elements to retard decomposition of the retained austenite and formation of embrittling phases in the austenitic matrix when the steel is reheated. In yet another form the invention resides in an austenitic manganese steel characterised by the addition of suffi¬ cient carbide forming elements to produce a dispersion of undissolved primary carbides in the austenitic matrix. O P s The various aspects of the invention will be better under¬ stood by reference to the following discussion when read in conjunction with the accompanying drawings wherein:- Fig. 3 is a graph showing the effect of alloy compo¬ sition on the structure of solution treated and quenched Fe-Mn-C alloys; Fig. 4 is a series of three graphs showing the iso¬ thermal characteristics of austenitic manganese steels (a) Fe-12.4Mn-l.28C (b) Fe-13.lMn-0.83C and (c) Fe-5.6Mn-l.22C; Fig. 5A is a graph showing the isothermal transforma¬ tion diagram for Fe-13Mn-4Mo-1.2C; Fig. 5B is a graph showing the age hardening charac¬ teristics Of Fe-13Mn-4Mo-1.2C; and Fig. 6 shows the variation of charpy impact values tested at 20°C after isothermal heating for one hour over a range of temperatures for Fe-13Mn-1.2C and Fe-14Mn-5Cr-0.6C. Criteria influencing the chemical composition range. 1. Effect of Carbon and Manganese The properties, wear resistance, toughness, castability, weldability impact resistance below 0°C and paramagnetism, are maximised in Fe-Mn-C alloys by maintaining a substan¬ tially wholly austenitic microstructure. The austenite phase in the Fe-Mn-C system is retained by rapidly cooling a critical range of alloy compositions from the solution treatment temperature. The critical carbon and manganese composition range is delineated in Fig. 3. However, in conventional manganese steel the retained austenite within this composition range is metastable and readily undergoes decomposition during isothermal heating at elevated temperatures or during continuous slow cooling from temperatures above the solvus line. The decomposi¬ tion products are Widmanstatten cementite, grain boundary carbide and pearlite which embrittle the manganese steel, decrease wear resistance and cause a loss in paramagne- tism. The rate of isothermal decomposition of the metastable austenite is dependent on the carbon and manganese con¬ tents. Fig. 4 illustrates that the decomposition of the retained austenite is retarded by:- (a) Increasing the manganese content. (b) Reducing the carbon content. An upper limit of 25% manganese is selected since the work hardening capacity (i.e. wear resistance) is substantially reduced beyond this level. In addition, the tensile yield strength drops below 300MPa at higher manganese contents and the melting point of the alloy is undesirably low. The carbon content of the retained austenite may be re¬ duced by one of two methods :- (i) Reduction of carbon in the overall composition of the alloy, for example from 1.2% to 0.6% carbon. (ii) Introduction of one or more strong carbide forming elements to the alloy to remove carbon from solution in the austenite. 2. Effect of Carbide Forming Elements (such as Chromium, Molybdenum, Vanadium, Tungsten, Titanium and Niobium) . There are five distinct advantages obtained by the addi¬ tion of strong carbide forming elements to austenitic man¬ ganese steel . 2.1 The formation of undissolved carbides removes carbon from solution in the austenite phase and effectively re¬ tards the decomposition of the retained austenite during reheating at elevated temperatures. The ability of the various strong carbide forming elements to remove carbon from solution is dependent on:- (a) Solubility of the various carbides in austenite. (b) Solution treatment temperature. (c) Stoichiometry of the undissolved carbides. (d) Atomic weights of the strong carbide forming ele¬ ments, 2.2 The presence of a fine dispersion of undissolved carbides in the retained austenite has the added advantage of increasing the wear resistance and the yield strength of the material. This method is superior to the "disper¬ sion hardening" technique sometimes practised on austeni¬ tic maganese steels since:- (a) Undissolved alloy carbides are harder than cementite. (b) Double heat treatment procedures are not required. (c) A higher volume fraction and more uniform dispersion of alloy carbides is obtained. (c) Very high, overall alloy carbon contents can be achie¬ ved without radical changes in σastability of the steel and with improved wear resistance. (e) A finer grain size occurs on solution treatment. By way of example, the alloy Fe-13Mn-5Ti-2.5C, solution treated and water quenched from 1050 deg C, exhibits a microstructure consisting of extremely hard (3000 HV) titanium carbide in an austenite matrix whose composition is similar to conventional austenitic manganese steel. 2.3 Supersaturating the retained austenite by the solu¬ tion of strong carbide formers at the highest solution treatment temperature has a number of additional advan¬ tages:- (a) The strong carbide formers are austenite stabiliers and effectively retain the austenite phase on rapid cooling over a wider carbon and manganese range shown in Fig. 3. (b) The decomposition of the retained austenite to em¬ brittling phases is retarded by high dissolution of strong carbide forming elements in the austenite phase. By way of example, compare the isothermal transformation characteristics of the alloy Fe-13Mn-4Mo-1.2C (Fig. 5A) with Fe-13Mn-1.2C (Fig. 4A). (c) The retained austenite phase, supersaturated with a strong carbide former, readily age hardens at iso¬ thermal temperatures below the solvus line by the precipitation of extremely fine carbides (Fig. 5B) . Such a structure will exhibit a higher yield strength than conventional austenitic manganese steels. 2.4 The carbide forming element chromium is highly solu¬ ble in austenitic manganese steels and its use will result in an increased corrosion resistance. Performance similar to the stainless steel alloys, is anticipated at high chromium levels. Furthermore, additions of up to 10% nickel are expected to further enhance the corrosion resistance of chromium rich, austenitic manganese steels and, at the same time, produce an extremely tough, high strength wear resistant, weldable grade alloy. 2.5 The hardness and yield strength of the retained aus¬ tenite is increased by solid solution hardening. This effect is illustrated by the influence of chromium content on solution treated hardness of the wholly austenitic structures of the following alloys:- Alloy Composition Hardness (HV10) Fe-10Mn-0.8C 186 Fe-10Mn-0.8C-2Cr 215 Fe-10Mn-0.8C-6Cr 239 Fe-10Mn-0.8C-10Cr 248 3. Weldability of Austenitic Manganese Steel The improved weldability of a modified austenitic man¬ ganese steel is illustrated by comparing of the effect of heat input on the impact properties relative to conven¬ tional austenitic manganese steels. The modified alloy Fe-14Mn-5Cr-0.6C was used for com¬ parison with reference material Fe-13Mn-l.2C. The adverse effects of heat input associated with the welding of these materials was simulated by isothermally heating at temperatures up to 900 Deg. C at approximately 100 degree Celcius intervals for a period of one hour. The heat treated alloys were subjected to charpy impact testing which is highly sensitive to the presence of embrittling phases. Test results are shown in Fig. 6. This data confirms metallographic observations illustrat¬ ing that the embrittling decomposition products, which readily form during the reheating of conventional austeni¬ tic manganese steel, are suppressed in the modified aus- tenitic manganese steel. Additional metallographic exami¬ nations of alloys isothermally heated over extended time periods indicate that the modified alloy is suitable for multiple welding operations (for example, repeated hard- facing by welding) without embrittlement. A further example of the invention was produced and selec¬ ted for investigation having a composition of 17% manga¬ nese, 6% chromium and 0.8% carbon. The steel was cast and prepared by solution treatment followed by water quench¬ ing. The decomposition characteristics of the alloy were determined by heating a number of samples of 550°C for periods of up to fifty hours. Metallographic examination of the thermally aged samples showed that the development of transformation reaction products which cause severe embrittlement of conventional austenitic manganese steel is greatly retarded in the austenitic manganese steel modified according to the example of the invention. In addition, wear resistance of the modified " austenitic manganese steel alloy was determined in a simulated high stress abrasion wear tester and the results indicated an abrasion wear rating similar to conventional austenitic manganese steel. The example of the invention (Fe-17Mn-6Cr-0.8C) illustra¬ ted advantages over conventional austenitic manganese steels as follows:- a. The critical rate of cooling required for the example steel on quenching from the solution treatment tem¬ perature is very much lower than that for conven- - UR£^- tional austenitic manganese steel. As a result, heavier section castings than hithertofore achieved may be readily produced and quenched without embrit¬ tlement. b. The reduced tendency of the modified alloys to form embrittling carbides on re-heating simplifies the welding of the austenitic manganese steel and extends its useful range of service applications. One specific application of the example of the invention where the example offers a distinct advantage over conven¬ tional austenitic manganese steel relates to use with gyratory cone crusher mantle and bowl liners which may weigh up to several tonnes each. These are manufactured from austenitic manganese steel and are rendered unsuit¬ able for service after a certain amount of wear has occur¬ red. There is now a trend to re-build these worn com¬ ponents by overlaying their surfaces with a series of hard facing weld deposits. These weld deposition layers may be of a mass in the order of tonne to an excess of 1 tonne and usually require a total welding period in excess of fifty hours. It has been demonstrated that worn, conventional austeni¬ tic manganese steel crusher components can be successfully re-built provided careful welding procedures are used and precautions are taken to prevent excessive heat input and temperature rise of the components during the welding process. However, since the effect of thermal ageing on welding conventional austenitic manganese steel is never entirely eliminated and the adverse influence is cumula¬ tive, there is an increasing risk of failure due to em¬ brittlement with each successive rebuilding of a com¬ ponent. The use of the steel alloy according to the invention in gyratory cone crusher mantle and bowl liners will simplify the re-building welding procedures and minimise the risk of embrittlement produced by repeated re-building. Other suitable compositions for specific purposes can be tailored within the scope of the invention. For example a steel comprising Fe-13Mn-7Mo and 0.8C suitably heat trea¬ ted exhibits improved hardness and yield strength as well as ability for continuous service at temperatures up to approximately 550°C whilst a composition of Fe-13Mn-10Cr-l.2C with up to 10% Ni exhibits improved corrosion and high temperature oxidation resistance. A composition of Fe-17Mn-6Cr-0.8C with up to 10% nickel exhibits improved impact properties at cryorgenic tempera¬ tures since the stress induced.

Claims: THE CLAIMS defining the invention are as follows:- 1. An austenitic manganese steel characterised by the addition of sufficient carbide forming elements to promote the retention of austenite during cooling from solution treatment. 2. An austenitic manganese steel characterised by the addition of sufficient carbide forming elements to retard decomposition of the retained austenitic and formation of embrittling phases in the austenitic matrix when the steel is reheated. 3. An austenitic manganese steel characterised by the addition of sufficient carbide forming elements to produce a disperson of undissolved primary carbides in the aus¬ tenitic matrix. 4. An austenitic manganese steel having an improved hardness yield strength characterised by the addition of sufficient carbide forming elements to produce a fine dis¬ persion of secondary alloy carbides in the austenite matrix by suitably age hardening procedures. 5. An austenitic manganese steel having an improved hardness yield strength characterised by the addition of sufficient carbide forming elements to cause solid solu¬ tion hardening the retained austenitic matrix. 6. An austenitic manganese steel as claimed in claim 1, 2, 3, 4 or 5 wherein the manganese content is within the range of 5% - 25%. 7. An austenitic manganese steel as claimed in claim 1, 2 , 3,4, 5 or 6 wherein the carbon content is within the range of 0.4% - 3.0%. 8. An austenitic manganese steel as claimed in any one of claims 1 to 7 wherein the carbide forming elements are selected from the group comprising chromium, molybdenum, titanium, niobium, tungsten and vanadium. 9. An austenitic manganese steel as claimed in any one of claims 1 to 8 wherein the quantity of carbide forming elements is determined by (i) solubility of the carbide in austenite, (ii) solution treatment temperature, (iii) stoichiometry of the undissolved carbides, and (iv) the atomic weight of the carbide forming elements. 10. An austenitic manganese steel as claimed in any one of claims 1 to 9 wherein the austenitic structure is formed by solution treating and quenching. 11. An austenitic manganese steel as claimed in any one of claims 1 to 9 wherein the austenitic structure is retained by solution treating and air cooling. 12. An austenitic manganese steel as claimed in any one of claims 1 to 9 including nickel . 13. An austenitic manganese steel comprising 5% - 25% manganese, 0.4% - 3.0% carbon and up to 20% of carbide forming elements, the balance being iron. 14. An austenitic manganese steel containing 13% manga¬ nese, 5% titanium and 2.5% carbon, the balance being iron. 15. An austenitic manganese steel containing 13% man¬ ganese, 4% molybdenum, and 1.2% carbon, the balance being iron. 16. An austenitic manganese steel containing 14% manga¬ nese, 5% chromium and 0.6% carbon, the balance being iron. 17. An austenitic manganese steel containing 17% man¬ ganese, 6% chromium and 0.8% carbon, the balance being iron. 18. An austenitic manganese steel containing 10% man¬ ganese 10% chromium and 0.8% carbon, the balance being iron. 19. An austenitic manganese steel containing 13% man¬ ganese, 7% molybdenum and 0.8% carbon, the balance being iron. 20. An austenitic manganese steel containing 13% man¬ ganese, 10% chromium 1.2% carbon and up to 10% nickel the balance being iron.

Inventors: DOLMAN KEVIN F, DOLMAN, KEVIN, F.
Applicants: VICKERS AUSTRALIA LTD, VICKERS AUSTRALIA LIMITED
Application year: 1983
Main classification: C22C38
IPC codes: C22C38