EFFECT OFHEAT TREATMENT ON THE MECHANICAL PROPERTIES AND WELDABlLlTY OF MARTENSITIC SliAlNLESS STE"t1L{16 Cr- 5 Ni-l Mo )

Abstract

A selected number of specimens from X4 CrNiMo 16-5-1 stainless steel were subjected to hardening treatment at 925 °C for 30 minutes followed by heating in the temperatures range ( 300 - 850 °C) for 4 hours . After heat treatment; tensile, hardness, impact, magnetic permeability tests as well as optical, scanning (SEM) and X-ray diffraction were conducted. The microstructure of all tempered specimens was found to consist of martensite with variable fractions of 8-ferri.te, and reversed austenite for samples tempered at 600 °C and 700 °C. The specimens tempered at 475 °C had maximum values of tensile strength and hardness with minimum values of ductility and impact toughness, this may be was attributed to the precipitation of Mo carbides. The specimens aged at 600 °C had maximum values of impact toughness and ductility. The softening of the martensitic matrix at this tempering temperature was due to the elimination of internal stresses, the decrease in the dislocation density, and high volume fraction of reversed austenite which lead to drop in values of tensile strength and hardness. Partial transfonnation of reversed austenite to untempered martensite may be the reason of the slightly reduction in toughness observed at temperature above 600 °C ( 700 °C ). However, no formation of sigma phase was reported when the samples were tempered at 850 °C.

Some samples were also tempered at 600 °C for different tempering time ( 1- 6 hrs ), After heat treatment; tensile, hardness, impact, magnetic permeability tests as well as optical and X-ray diffraction were conducted. The microstructure of all tempered specimens was found to consist of martensite with variable fractions of 8-ferrite and reversed austenite. Samples. tempered for 6 hrs shows the highest toughness and ductility, and lowest strength and hardness, this is because; the maximum amount of reversed austenite is present at this condition.