EFFECT OF MICRO ALLOYING ON MECHANICAL PROPERTIES AND WEAR CHARACTERISTICS OF HADFIELD MANGANESE STEEL

Abstract

Austenitic Mn steel, known as Hadfield steel, has a nominal composition: 1.0-1.4 C (wt. pct.), 11.0-14.0 Mn, 1.0 Si (Max) and 0.07 P (Max). This steel with its extremely high strength and toughness possesses high work hardening capacity and resistance to wear. Although this steel has been used for nearly a century since its development in 1882, the mecha-nisms of rapid work hardening and of wear remain unclear. No systematic study has been made to correlate the wear charac- teristics with work hardening tendency. It is also observed that no systematic attempts have been made to study the role of various micro alloying elements in Hadfield Mn steel to improve casting characteristics, grain refinement, mechanical properties, and wear characteristics. In the present investi-gation the effect of micro alloying of Hadfield steel by B, V and cb has been studied on the grain refinement, mechanical properties, work hardening and wear characteristics. The micro alloyed Hadfield steels using four concentrat-ions of B in the range of 0.001 to 0.004 pct. and three concen-trations of V and Cb each in the range of 0.05 to 0.50 pct. were prepared by induction melting. The pouring temperature of liquid was kept within 1500 ± 10°C in all the compositions. Test samples of appropriate dimensions were poured for carry-ing out tensile, impact, hardness and wear tests. All tests were conducted after a standard heat treatment, which included austenitising at 1050°C for 90 minutes followed by water quench-ing. Light microscopy and scanning electron microscopy were extensively employed to study the work hardening and wear mechanisms under different test conditions. Micro alloying additions have been observed to signifi-cantly affect the tensile properties. For example, addition of 0.003 pct. B (0.0038 alloy) enhances the yield strength (YS) of base steel (13 Mn alloy) from 392.4 to 439.0 MPa ulti-mate tensile strength (UTS) from 650.4 to 740.4 MPa and pct. elongation (El) from 20.0 to 35.7 pct. Micro additions of V at lower concentrations (0.05 V and 0.10 V alloys)also enhance YS, UTS and El but there is marginal drop in El in higher con-centration (0.50 V alloy). With additions of Cb, the YS and UTS show an improvement, but the El is reduced. Higher _concen-tration of Cb (0.50 Cb alloy) also shows redution in UTS in comparison to lower concentrations. The strain hardening exponents (ri ) have been determined from the true stress-true strain diagrams in the plastic range derived from tensile curves. It is observed that all the micro alloying additions increase the strain hardening exponents of the base steel, the effect of V addition being maximum.' The load-elongation curves under slow strain rate (3x104 Sec-1 ) show serrated flow which are also maximum in amplitude and in frequency in 0.10 V steel. The serrations are directly related with the value of strain hardening exponents. The pre-sent investigations have shown that work hardening characteris-tics during tensile loading differ significantly from those observed during impact loading. The micro alloying of Hadfield Mn steel by B, V and Cb has been observed to affect significantly the impact strength. Maximum improvement in impact value is observed in case of 0.003 B'alloy. Additions of 0.05 and 0.10 pct. V marginally improve the toughness value, whereas 0.5 pct. V addition causes deterioration in the impact value. Toughness is significantly reduced by Cb additions. From the results obtained, it is observed that 0.003 B alloy gives the optimum mechanical properties out of four different concentrations of B selected for experiments. Simi-larly 0.10.v alloy gives higher YS and UTS without affecting ductility and toughness of base alloy (13 Mn). However, 0.50 Cb alloy shows maximum improvement /in YS though elongation and toughness are adversely affected with all other concen-terations. Hence these three compositions (0.003 B, 0.10 V and 0.50 Cb alloys) were selected for further investigation on work hardening and wear characteristics. -iv- All the Hadfield steels investigated exhibit significant work hardening capacity under impact loading. The maximum im-provement in hardness is observed in 0.003 B steel. The 0.10 V alloy shows almost the same hardness level as of base steel; whereas there is decrease in hardness level with Cb addition. The saturation in hardness (486 VHN) is attained very rapidly (50 blows) in 0.003 B alloy in comparison to all other compo-sitions studied in the present investigation. The depth of hardening was studied for all samples sub-jected to impact loading. A sharp drop occurs in the hardness below the impacted surface upto a depth of 0.3-0.8 mmibeyond which the hardness attains a constant level which is about 60 to 140 VHN higher than the as quenched hardness. Most predo-minant effect is observed in the 0.003 B alloy. In this alloy the thickness of variable hardness below the impacted layer is only 0.30 mm beyond which a constant hardness level (350 VHN) is attained which is about 140 VHN higher than as quenched hardness. The wear resistance, as measured by weight loss, for various Hadfield compositions was studied under conditions of impact-slide and slide ( by grinding on emery wheel). The 0.003 B alloy shows remarkable improvement in wear resistance under both conditions. additions of V and Cb have been observed -v- to deteriorate marginally the wear resistance of Hadfield steel. The present investigations have revealed that the wear resis-tance of Hadfield steel is directly linked to its saturation hardness under impact-slide wear conditions and has higher wear resistance in comparison to only slide wear conditions. The wear resistance, as determined after grinding off the vari-able hardness layer, closely corresponds to the wear resis-tance of as quenched, unimpacted samples. It, therefore, app-ears that the nature of the variable hardness surface layer formed on impact loading plays a vital role in enhancing the wear properties of Hadfield steels. It is also established that in addition to its relationship with saturation hardness, the wear resistance also is governed by metallographic structure and other mechanical properties like toughness, UTS and' El etc. The 0.003 B alloy, which shows maximum improvement in wear resistance in all conditions, is also observed to possess maximum toughness and higher UTS and El among all the compo-sitions investigated. Metallographic ,studies reveal that all micro alloying additions cause refinement in the as cast grain structure. The refinement is almost of the same order with all micro alloy-ing compositions. It has been observed that there is no altera-tion in the grain structure by heat treatment except that there are certain structural changes. The addition of B results in an austenitic structure almost free of carbides relative to base composition (13 Mn alloy). The 0.10 V alloy shows some -vi- fine precipitates of V4 C3 at grain boundaries and within the grains. The Cb addition results in the formation of carbides mainly on the austenite grain boundaries. The presence of these carbides is observed to significantly affect the mechanical properties, work hardening and wear characteristics. Light microscopy and Scanning Electron Microscopic(SEM) studies on impacted samples reveal the formation of cracks around these particles indicating decohesion at the carbide-matrix interfa-ces. Formation of twins has been observed in all the samples which is the principal cause of.workhardening under impact loading. Maximum density of twins has been observed in 0.003 B alloy, whereas 0.50 Cb alloy shows minimum concentration of twins. SEM studies on fracture surfaces of impact tested samples re-veal extensive elongation of grains before seperation in the case of 0.003 B alloy indicating a high toughness of this steel. Tensile fracture of 0.003 B steel indicate a fine net work of small size. dimples which are formed around closely spaced striations. This indicates the higher ductility and deformation characteristics of 0.003 B alloy. Formation of fine cracks at carbide matrix interfaces have been observed in Cb and V bearing compositions which lead to the ultimate failure. Ex-tensive SEM studies on wear surfaces at various stages of impact-slide wear reveal that abrasion is accompanied by simultaneous plastic deformation (micro-ploughing and micro-cutting) and brittle fracture (micro-cracking). The deformation is maximum in 0.003 B alloy due to its higher elongation and toughness, whereas it is relatively less in 0.10 V and 0.50 Cb alloys. In V and Cb bearing alloys, micro cracks are observed to form at the interfaces of carbide particles and matrix which further propagate resulting in material removal in the form of relati-vely bigger flakes showing poor wear resistance. This observa-tion was also confirmed by the size of debris formed in various micro alloyed compositions. Very fine sized debris is formed in 0.003B alloy in comparison to other compositions. Thus the SEM studies have confirmed the significant improvement in wear resistance by B additions and a deterioration by V and Cb additions. The entire work reported ih this thesis has been spread over six Chapters. Chapter 1 deals with the critical review of the available literature on Hadfield Mn steel. This includes the structural characteristics of the alloy, heat treatment, various mechanisms to explain work hardening and wear characteristics. The effect of various alloying elements in the nominal range and some of micro alloying elements on mechanical properties is criti-cally reviewed. In Chapter-2 the problem of the present investigation has been formulated and posed on the basis of the perspective of present investigation. Chapter 3 describes details of experimental set-up; alloy and sample preparation; method and machines used for measuring the various properties and corresponding micro-struc-tural changes. The details of various microstructural characteristics, mechanical properties viz. tensile properties, impact strength, hardness, effect of impact loading on hardness and wear pro-perties under various test conditions of base alloy and micro alloyed compositions have been presented in different sections of Chapter 4. The effect of micro alloying additions on mechanical properties, work hardening tendency and wear characteristics observed in the present investigation is critically discussed in Chapter 5. Conclusions and suggestions for further work in this field of study are given in Chapter 6.