STRUCTURE-PROPERTY RELATIONS IN 10Mn-3C-7Cr WHITE IRONS ALLOYED WITH I.5-5.0 COPPER

Abstract

[Al Background Of the three varieties of corrosion resistant alloy cast irons in use, the high Si irons have useful applications only in strongly oxidizing conditions. They however, suffer from poor mechanical strength and shock resistance. The high nickel irons, although extensively used in a number of aqueous environments, have a low strength, suffer from graphitic corrosion & pitting and are unsuitable at operating temperatures ~800C, The high chromium irons exhibit relatively higher strength and can be employed ' upto higher service temperatures. Their shock resistance is improved by lowering carbon content. A critical analysis revealed that little information is available on the structure-property interrelations in alloy cast irons in general. Furthermore, there is a lack of systematic information on the electro-chemical and on the deformation behaviour of microstructures commonly encountered in alloy white irons namely, 'martensite + carbide'(M + CY, 'austenite + carbide'(A + C), and their allied counterparts. Detailed information on these aspects was likely to prove useful in ascertaining whether microstructures exhibiting good resistance to aqueous corrosion and useful mechanical properties could be attained through the 'white iron' route. A major advantage foreseen was that the limitations encountered in alloyed gray Irons would stand eliminated. Equally pertinent would be to investigate whether these microstructures could be generated by utilizing low cost alloying elements(Mn, Cu etc.) in preference to the conventionally employed costlier alloying elements Ni and Mo. Work carried out at the University of Roorkee by Jain and Kumar under the supervision of Professor A.K.Patwardhan has demonstrated that new meaningful compositions with very good corrosion resistance and deformation behaviour could be designed/developed based on the Fe-Mn-Cr-Cu system. The data thus obtained, while affirming freedom from the drawbacks encountered in the existing grey irons currently in use, laid down guidelines for d-ev-eluping future alloy compositions with considerably improved properties with the eventual interest in developing a new . generation of corrosion resistant cast irons. The inferences arrived at mainly stressed upon the stability and volume fraction of austenile, volume fraction, morphology, and compatibility of the massive carbides(MCs) and size, volume fraction, and distribution of dispersed second phase(DCs)-an unintended constituent in attaining desired 'end properties'. These guidelines were used in conceiving and designing new alloys which were investigated in detail in the present study. [B] Present Investigation The present study, essentially comprised a detailed investigation of certain newly designed Fe-Mn-Cr-Cu white iron compositions, namely, Fe-3C-lOMn-7Cr alloys containing 1.5, 3.0, and 5.0%Cu in the air cooled condition. It centered around assessing their heat treatment response aimed at establishing an interrelation between structure and properties, A study of this kind required a detailed insight into the transformation characteristics of the alloys. This aspect accordingly received x maximum attention in the present study. The alloys which were air induction melted and sand cast (18mm round and 120x22x8mm rectangular strips), were investigated for arriving at their transformation behaviour, by employing hardness measurements, optical and scanning metallography, quantitative metallography, X-ray diffractometry, electron probe .micro analysis and differential thermal analysis. The electro- chemical characterization of the alloys was carried out by employing the weight loss/potentiostatic methods. Compression testing was also carried out to assess the deformation behaviour of the experimental alloys. Computational techniques were extensively employed for data analysis using IBM compatible PC-XT and PC-486 systems. Necessary software packages were also developed in FORTRAN IV as and when required. [C] Major findings and development of models The experimental work involved subjecting round specimens of Lite three alloys P1, P2, and P3 to heat treatments comprising holding for 2, 4, 6, 8, and 10 hours at 800, 850, 900, 950, 1000 arid 1050'C followed by air cooling. This treatment was preferred over oil quenching because it can be directly utilized for industrial applications. Optical metallography was extensively used to assess how the Cu content and heat treating schedule influenced the microstructure which comprised (i) Austenite (A) + some martensite (M)(?) + MC in the as-cast state, (ii) A + MC + dispersed second phase (DSPs) on heat treating P1: 7Cr-l0Mn-1.5Cu; P2: 7Cr-10Mn-3.0Cu; P3: 7Cr-10Mn-5.00u x. i from up to 950C, (iii) A + MC (mostly rounded/hexagonal) on heat treating from upto 1000C, and (iv) A + large agglomerated MCs + some dispersed carbides (DCs) on heat treating from upto 1050'C The volume fraction of massive carbides(MC) decreased with temperature or with soaking period at a given heat-treating temperature, the effect being marked at temperatures 1950C. Simultaneously, massive carbides were rendered discontinuous from the 'early' stages of heat treating. The 'rounding-off' tendency set in even at 9000C. Dispersed second phase (DSPs) formed on heat treating at 800C directly from austenite and comprised to begin with mostly needle/plate shaped precipitates and some DCs. With an increase in temperature/time the needles and the DCs coarsened, needle ends spherodized & slowly, the coarsened needle assumed the shape of a hexagonal/rounded massive carbide (MC). The extent of coarsening which was marked at 900'C and 950'C, has been represented by a newly evolved parameter the 'coarsening index'(CI). The dispersed carbides dissolved on heat treating from 1000'C but reappeared on heat treating at 1050C. Hardness measurements provided a quick yet reliable indication of the mechanical properties. A model was developed interrelating heat treating temperature and time on hardness and is of the form H = C1 eC2/T + (C3 + C4.T).t where, H = hardness, VHN3o T = temperature, °K t = time in seconds Cl, C2, C3 and C4 are constants and are different for different alloys. Through intensive calculations it has been possible to demonstrate that the first term of this model represents the matrix related transformations and the second term represents the :carbide' transformations. The model is thus physically consistent. The predicted hardness values are within ±5% of the experimentally determined values. 3D plots interrelating the hardness-heat treating temperature & time were also constructed to study the overall transformation behaviour at a glance. The plots revealed that the above said relationship can be represented by a surface with opposite slopes on the two sides of the temperature axis. The aforesaid model(hardness--temperature-time) was based on a total of 30 experiments. Through 'modelling' it has been demonstrated that the transformation behaviour can be simulated with equivalent accuracy based on merely 4 or 6--experiments. This is an important inference needing further exploration since the new idea put forth could greatly help in minimizing arduous experimentation in arriving at the transformation behaviour. The variation in volume fraction of MCs and DCs for a given heat treatment has been utilized to evolve a new concept called the homogeneity/heterogeneity index. It is felt that this concept needs to be further enlarged upon to arrive at its fuller implications. X-ray diffractometry proved extremely helpful in identifying the different micro-constituents observed in the experimental alloys (both in the as-cast arid in the heat-treated conditions). It proved helpful in identifying the,Matrix microstructure in 'marginal' cases e.g. in confirming the presence of martensite(M) in the as-cast condition even when the matrix was by and large austenitic. It also established that amongst the likely carbides to be present namely M3C, M23C6, M5C2 and M?C3, only•M3C & M7C3 were attained in the as-cast condition as well as formed in differently identified temperature regimes used in the present study. Additionally, presence of Cu in the elemental form and of 'Fe--Si-carbide(Fe8SiZC) was also established. Even after such a detailed analysis, carried out with the help of developed software packages, certain reflections remained unidentified whose indexing was possible on the likely formation of CrMn3 and Cu2S. This aspect needs further investigation. EPMA studies carried out on as-cast as well as heat treated specimens besides confirming the deductions arrived at on the basis of X-ray diffractometry arid optical metallography, helped in establishing the partitioning behaviour of the different alloying elements e.g. Mn, Cr and Cu into the matrix and carbide phases. Chemical composition of the MCs and the DSPs was also determined and this alone helped in establishing the true identity of the different types of MCs & the DSPs, 'haloed' regions forming around MCs, and the dark etching regions in between adjacent MCs. EPMA also confirmed the (i) existence of Cu enriched regions both in the as---cast and heat treated conditions xiv and (ii) presence of M7C3 and M3C from amongst the different carbides likely to be present. Differential thermal analysis(DTA) of the experimental alloys in the as-cast condition revealed that all the alloys underwent transformations at 540-560'C(matrix related transfor- mation) and =940-990'C(carbide transformation). Additionally, the alloy P1 underwent a third transformation.at 1020'C repre-senting perhaps another carbide transformation. The same study also proved useful in predicting the suitability of the experimental alloys for high temperature applications based on an analysis of Lhormogravimetric(we*fight gain) data. The as-cast microstructures were found to be suitable up to a service temperature of 800C. This beneficially reflected upon attaining a microstructure, normally observed at high temperatures, down to room temperature for improving the high temperature performance of the alloys. On heat treating from 1000C, the temperature limit had been raised to 950'C. A mathematical model, developed to interrelate the weight gain with temperature, is of the form % TG = At + A2.e(-A3/T) where, % TG = percent weight gain T = temperature Al, A2, and A3 are constants. Weight loss studies, carried out in 5% NaCl solution, were helpful in characterizing the alloys/selected microstructures for their response to corrosion. Corrosion data of two Ni-resist compositions were also considered for the purpose of a comparison. The study clearly brought out the effect of the xv second phase(MC + DC) namely the morphology and volume fraction of the MC and the size, shape and distribution of the DC in controlling corrosion e.g. plate like morphology and a large volume fraction of the MC had an adverse effect in spite of the austenite matrix being favourably disposed in improving corrosion resistance. Similarly a favourable morphology of MCs (1000'C, 10 hrs and 1050'C, 10hrs treatments) reducpsthe adverse effect. Heat treating between 900-950C adversely affected corrosion resistance due to the presence of needle shaped DSPs and also due to diverse nature of the DSPs present(needle and spherical particles) and matrix heterogeneity. Interestingly, all the three alloys on heat treating from 1050'C(10 hours heat treatment) attained corrosion rates comparable to those attained in standard SG/Flake graphite Ni-Resist compositions. This analysis has enabled laying down of guide lines for developing improved corrosion resistant microstructures.