Abstract
The problem of weld cracking in austenitic stainless steels is introduced together with a survey of work carried out to try and explain the phenomenon. It is clear from this survey that although the problem is solvable by allowing a small amount of ferrite to appear in the final weld microstructure the detailed reasons for the cracking are still unknown - especially in the particular case of cast to cast variations. This thesis has looked at 6 alloys which are similar in composition, all bar one of which meet the AISI 316 specification. The initial premise was that some of these alloys would be easy to weld and that some would be prone to cracking. Experimental modelling (using a Quenched Directional Solidification or QDS technique) has been used to look at the detail of how the alloys solidified, with respect to the thermal properties, fraction solid against temperature, phase chemistry and microstructure. Sophisticated techniques much as electron probe microanalysis and computer controlled image analysis have been used to extract the results from these experiments. These models have been correlated with a small amount of weld testing using the 'Varestraint' process. The microstructural results revealed that all the alloys initially solidified as ferrite and underwent a transition to austenitic solidification as the temperature dropped. The transition between the two solidification modes varied in its suddenness - this appeared to be a function of the impurity content of the alloy. Analysis of the EPMA results showed that both eutectic and peritectic transformation reactions apparently occurred. The Clyne-Davies model of solidification cracking has been applied to experimentally determined fraction solid curves. This method provided some predictive insights and could usefully be more widely applied in technological cases to predict if a particular cast of material may be prone to weld cracking.