published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

A dissertation submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering. ,Department of Metallurgy and Materials Engineering University of the Witwatersrand Johannesburg, 1990. / An instrumented hot deformation simulator was desLgned, cons'.:ructedand commissioned at Iscor's Pilot Plant. A modified servohydraulic machine, using plane strain compression, simulated industrial plate rolling schedules. The simulation test included induction heating, multiple pass plane strain compression and either air cooling or quenching. The movement of the specimen between the different test stages was computer controlled. Accurate control of specimen temperature, strain atrain rate and interpass hold times was achieved by means of sophisticated data acquds i+Lon equipment. A comput er programme was written to simulate the hot. rolling of plate in t erms of roll sepa ratIng f.orces, ffiE.\tallurgical changes during deformation as well as the final microstructure and mechanical properties of SGa carbon manganese steel. Multiple linear regression was pe.rformed on the results obtained from mUlti-pass plane strain compression tests. It was found that themical composition, finish temperature and finish strain were the most important process parameters affecting yield strength and impact energy of air cooled place. A computer model was developed to simulate the temperature distribution in the deformation zone of a plane strain compressLon specimen at any point during or after a mUlti-pass test.

The effects of hydrogen on the fracture behavior of manual SMA welds in carbon steel plate was investigated utilizing modified ½T compact tension specimens. Tests performed on these specimens in the presence of hydrogen were compared to similar tests in helium. These tests showed that hydrogen lowers J_C in both the heat affected zone and the base metal. In 350 psi helium, the experimental value of J_C in the heat affected zone (2826 in.-lbs./in.²) was greater than that obtained in the base metal (1650 in.-lbs./in.²). The tests conducted in 350 psi hydrogen resulted in a reduction in J_C for both the heat affected zone (1425 in.-lbs./in.²) and the base metal (59 in.-lbs./in.²). Furthermore, when compared to specimens tested in helium, it was determined that the material tearing modulus for specimens tested in hydrogen was significantly reduced. Slow stable crack growth occurred in all helium tests and in tests performed on the heat affected zone in hydrogen. However, unstable crack growth (fast fracture) was obtained for base metal tests in hydrogen. Fractographic studies revealed that the mechanism for all slow stable crack growth was microvoid coalescence; whereas, the surface of base metal specimens tested in hydrogen showed that fast fracture occurred by cleavage. Optical microscopy revealed that the fracture path for all base metal tests remained in the base metal, but that the fracture path for all heat affected zone tests moved towards the base. These observations, in conjunction with microhardness readings and quantitative metallography, were used to develop explanations for the observed behavior. These explanations include the combined effects of hydrogen, weld defects, residual stresses, grain size, and test variables such as temperature and specimen size and geometry. / M. S.