An Introduction to Metallurgy 2nd Edition PDF

Introduction

Since the pioneering studies on continuous cooling transformation (CCT) diagrams carried out by Christenson et al:¹ were published almost 50 years ago, many hundreds of CCT diagrams have been constructed throughout the world to describe the γ-α transformation kinetics of most grades of commercial steels. Because most of the metallurgical processes occurring in steels involve continuous cooling before the final microstructure is obtained, the use of CCT diagrams to present the "γ-α transformation reactions has obvious practical advantaget!smpared with other methods such as the well-known time temperature transfor tion (TTT) diagrams.

The initial CCT diagrams were constructed for wrought steels and these cannot usually be directly applied to the cooling cycles experienced in welding situations. By employing modified reaustenitising procedures, the method was applied to the weld ability of steels and consequently, several CCT diagrams applicable to the coarse grained region of the weld heat affected zones (HAZ) were published.^2-8 However, since mid-1970s, increasing demands for weld metals of high toughness at low temperatures with the appropriate microstructures has produced the requirement for a more systematic and detailed study of transformation kinetics and mechanical properties of low alloy weld deposits. This resulted in a number of CCT diagrams which were directly applicable to weld metals and these have significantly improved our understanding of weld metal microstructural development and the effects of different factors, such as chemical composition, oxygen content (thus size distribution and population of inclusions), welding parameters (e.g. cooling rate) and prior austenite grain size, on the γ-α transformation behaviour of weld metals.^9-24

It is therefore of both practical as well as academic importance to draw together an atlas of CCT diagrams applicable to low carbon low alloy weld metals. It is hoped that these diagrams will be of assistance to welding engineers, welding metallurgists, welding-consumables designers in industry. At the same time, they will also prove useful to those in academia who are involved into investigations of steel weld metal phase transformation kinetics.

Edited by: Z. Zhang, R.A. Farrar