Susceptibility to and occurrence of HAZ liquation cracking in rail steels: Study of rail welding with high-C welding materials (4th Report)

Summary

The conventional enclosed manual metal arc welding process used for field welding rails has the disadvantage that liquation cracks are likely to occur at the boundaries of coarsened austenite grains in the heat affected zone.

This paper describes an investigation of the HAZ liquation crack susceptibility of rails and the causes of cracking. Experiments have been conducted to determine the critical C content of the weld metal and critical welding conditions necessary to avoid cracks by use of electrodes with various C contents. Synthetic tests involving HAZ liquation cracks being simulated have also been conducted to compare crack susceptibility and causes of cracks in rail steels with the corresponding behaviour of general structural steels. The experimental results obtained may be summarised as follows:

1. The crack incidence increases with a decreasing weld metal C content when the difference in the C content between the weld metal and rail steel is 0.3% or more. Cracking is prevented when this difference is less than 0.2%.

2. The crack susceptibility increases with an increasing electrode diameter and welding current or welding heat input.

3. The HAZ liquation crack susceptibility of presently marketed rail steels is equivalent to that of mild steel not facing a HAZ liquation cracking problem in terms of steel chemistry.

4. The principal cause of HAZ liquation cracking in rails welded by the manual metal arc welding process is the large difference in the liquidus and solidus temperatures due to the difference in C content between the weld metal and base metal.