Influence of Austempering and Quenching and Partitioning (Q&P) Heat Treatment on the Mechanical Properties and Wear Behavior of AISI 9255 Steel

Abstract

The present investigation deals with the influence of two different heat treatments namely: (i) Austempering and (ii) Quenching & Partitioning (Q&P) on the microstructure, mechanical properties and dry sliding wear behavior of AISI 9255 high silicon steel. The as-received steel was checked for microstructure that comprised of pearlite, pro-eutectoid ferrite and undissolved cemetite, Mechanical properties mainly tensile strength of 800 MPa, % elongation of 14.1 and hardness of 334 HV. Specific wear rate was found to be 4.076 ×10-5 mm3/N-m. Austempering was carried out on the test samples by initial austenitisation at 900 oC for 45 minutes and thereafter quenching to various temperatures from 280 to 400 oC and holding for varying lengths of time from 15 to 180 minutes, which generates bainite microstructure that mainly comprises of bainitic ferrite and retained austenite (RA). Accordingly, variation in the bainitic morphology from acicular (lower bainite) to lath (upper bainite) was observed with the increasing temperature from 280 to 400 oC. However, increase in austempering time for all temperatures revealed a bit densely packed bainite structure. As a result there was a decrease in the RA; the quantity of decrease was further confirmed by x-ray diffraction (XRD) analysis. When compared to the properties of as-received steel, significant improvement in the tensile properties was observed for all austempered specimens; superior combination of tensile strength and % elongation was attained at austempering time of 15 min, i.e. (1852 MPa & 14%) at 280 °C, (1155 MPa & 33.4%) at 360 °C and (1165 MPa & 34%) at 400 °C. Similarly decrease in hardness from 600 to 342 HV was observed with increase in austempering temperature. During tensile loading, there was transformation of blocky RA to strain induced martensite that resulted in excellent strain hardening response from the samples austempered at higher temperatures 360 and 400 oC. Wear test results showed the slight increase in specific wear rate as the austempering time increases from 15 to 180 minutes. The sample austempered at 280 °C for 15 minutes exhibited the least specific wear rate of 2.063×10-5 mm3/N-m implying its superior wear resistance. In another set of heat treatment experiment i.e. quenching and partitioning was carried on the test samples by initial austenitisation at 900 oC for 45 minutes followed by quenching to 190 oC and finally partitioning at various temperatures from 280 to 400 oC and holding for varying lengths of time from 15 to 90 minutes. This heat treatmentgenerate multiphase microstructures mainly consisting of martensite of both lath and plate-type, transitional ε-carbides in tempered martensite matrix, lower bainite and RA for all the conditions. At higher partitioning temperatures i.e. 360 and 400 oC reveals some bainitic ferrite laths along with martensite and RA. Quantitative analysis shows the decrease in RA content with increasing partitioning time at 280 °C while it remains more or less constant for other partitioning temperatures. RA was non quantifiable for the samples which have undergone prolonged partitioning times at higher temperatures. Electron back scattered diffraction (EBSD) phase distribution maps reveals the RA content quite closer to those measured by XRD. Martensite crystals in inverse pole figure (IPF) maps reveal more of plate like morphology along with some twin orientations. Misorientation profiles indicated that at lower partitioning conditions i.e. 280 °C for 15 & 90 minutes reveals more high-angle grain boundary (HAGB), which signifies the presence of lower bainite in the matrix and similarly at high partitioning condition i.e. 400 °C for 30 minutes shows more lowangle grain boundary (LAGB) indicating the presence of upper bainite along with martensite of various morphologies, transition ε-carbides and RA. Compared to the mechanical and wear properties of as-received steel and austempered specimens, the Q&P treated specimens exhibits higher tensile strength and hardness but without much improvement in % elongation for almost all partitioning conditions. An excellent combination of tensile strength and % elongation was attained at partitioning time of 15 minutes i.e. 1859 MPa and 17% at 280 °C, which was also associated with high hardness value of 660 HV. Specific wear rate shows marginal increase with the increasing partitioning time from 15 to 90 minutes for all temperatures from 280 to 400 °C. The least specific wear rate of 1.18×10-5 mm3/N-m was obtained at a partitioning condition of 280 °C for 15 minutes which is much less compared with that of austempered condition, which was in turn influenced by presence fine martensite packets (lath and plate morphologies), lower bainite along with stabilized RA.