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Achieving Lightweight, Wear Resistant Steels Through Nitriding


A low cost heat treatment method could improve the wear resistance of austenitic manganese and aluminum steels. LAURA BARTLETT AND SABRA SERINO, TEXAS STATE UNIV., SAN MARCOS, TEXAS


intensely investigated in recent years for their potential applications in military and transportation industries. Tese steels have exceptional combina- tions of high strength and toughness with excellent wear resistance. Adding aluminum in levels from 6% to 8.8 wt% reduces the density by 10% to 15% compared with quenched and tempered chromium and molybdenum steels but also decreases strain harden- ing and abrasive wear resistance. Wear resistance may be improved by a low- cost case hardening heat treatment


H


Steel A Steel B Steel C


Steel A Fe C Si


igh manganese and aluminum austen- itic steels have been


in a nitrogen atmosphere (nitriding) that produces a hard surface layer of aluminum nitride, AlN. In the current study, the effect of aluminum and silicon content on the kinetics of the nitriding process was evaluated for a Fe-30%Mn-(6-9%) Al-(1-1.6%)Si-0.9%C steel in the temperature range of 1,652-2,012F (900-1,100C). (Note: all following per- centages are given in weight percent). Results showed that up to a 550 µm thick surface layer of aluminum nitride plates could be produced, depending on the time and temperature. Increas- ing the amount of silicon from 1.1% to 1.6% had no statistical effect on the


Table 1. Chemical composition of alloys in weight percent Alloy


Mn


Bal. Bal. Bal.


0.90 0.89 0.94


1.07 1.56 1.57


30.42 29.97 30.22


P


0.001 0.002 0.001


Table 2. EDS chemistry of microconstiutants in weight percent Steel


Phase AlN


Steel A Matrix between AlN plates Steel C


AlN


Steel C Matrix between AlN plates Steel A Steel B Steel C


Oxide layer Oxide layer Oxide layer


28 | MODERN CASTING June 2015 Treatment


1000°C 6hrs 1000°C 6hrs 1100°C 8hrs 1100°C 8hrs 900°C 8hrs 900°C 8hrs 1100°C 8hrs


Fe


Bal. Bal. Bal. Bal. Bal. Bal. Bal.


S


0.006 0.007 0.012


Mo


0.54 0.53 0.61


diffusion of nitrogen in the temperature range of 1,652-2,012F (900-1,100C). However, increasing the amount of aluminum from 6% to 8.8% decreased the depth of the AlN surface layer and increased the calculated activation energy for the diffusion of nitrogen in austenite from 64 to 79 kJ/mol. Tese calculated activation energies can be compared to a value of 168 kJ/mol for bulk nitrogen diffusion in pure austen- ite same temperature range. Te lower than expected calculated values of the current study is most likely the result of the development of high diffusivity pathways at the interface between the aluminum nitride and austenite matrix.


Ni - -


0.011 Al


8.83 8.81 5.98


Cu


0.006 0.006 0.010


Si -


2.0 -


1.3 2.4 5.4 -


Mn -


54 -


53 36 30 17


O


< 4 -


< 4 -


31 36 40


N


31 -


34 -


5.8 - -


Al 64


0.3 63


0.3 14 23 33


Mo - - -


0.51 - - -


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