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Abstract

The influence of tempering on the impact and tensile properties of various types of microstructures has been examined. The following heat treatments were investigated during the course of this project.

• The structure property relationships for two quenched and tempered C- Mn-Al steel flanges made to ASTM A350 LF2 specification have been determined. Samples heated to temperatures in the range 900 - 1250°C to produce a wide range in 7 grain size and quenched in oil or iced water followed by tempering at 300, 600, 650 and 700°C to produce a variety of commercially obtainable micro-structures. Impact transition curves as well as the tensile strength and the Vickers hardness values were obtained. Increasing the cooling rate and quenching temperature increased the hardness and strength.

The microstructural parameters have been determined and related to the Vickers hardness, the yield stress, and impact behaviour. The re-sults obtained have been analysed and it has been found that 7 grain size is the major structural parameter controlling the mechanical prop-erties of these quenched and tempered flanges.

Tempering improved the impact behaviour of quenched steels while the strength and hardness fell. The fall in strength and hardness was found in part to be related to a reduction in dislocation density. How-ever spheroidization of carbides and a reduction in the ky value after tempering are also possible reasons for the further fall in strength. The improvement in impact behaviour, however, is associated with the fall in strength and coarsening of the grain boundary carbides.

• A V free and a V containing steel flange have been examined and the impact and tensile behaviour determined after normalising at 900° C and austenitising at 1050°C . After normalising, both steels had fine grain sizes and passed the ASTM A350 LF2 specification. Austenitising at 1050°C coarsened the grain size and produced a large amount of precipitation hardening in the V steel. However, because of its low S level and higher shelf energy the V steel met the impact requirement, while the V free steel in which there was no precipitation hardening failed the specification.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TJ Mechanical engineering and machinery
Departments:	School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses School of Science & Technology > Engineering

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