Published on 28 January 2013
Through the combination of the specialist expertise in metallurgy, materials, fabrication and performance and its access to cutting-edge technology, TWI have the facilities and knowledge for assessing microstructure in order to optimise fabrication methods, establish the performance or select and qualify materials.
Case study: A 20C-10Co-5Mo steel containing 0.005%C had been diffusion bonded. Thermo-Calc was used to predict what influence would have diffusion bonding above 1000◦C on the microstructure and thus inferred whether mechanical properties could be recovered.
Experimental Approach: the phase transformation and proportion was established as function of the temperature in order to determine the opportunity for a grain size refinement heat treatment after diffusion bonding. The results were employed to optimise the process and find bounding parameters compatible with the mechanical properties.
Findings: The material is fully austenitic at 670◦C and above; and at ambient temperature approximately 30% austenite will be present in the material (versus 63% ferrite/martensite). Diffusion bonding would result in a very large prior austenite grain size (well into the single phase region). Some proportion of grain refinement might be possible by using a re-austenising annealing heat treatment above 700◦C, but this may be hindered by the residual austenite effectively giving a memory effect for austenite grain size. Several re-austenising annealing cycles might be necessary to produce significant refinement.
For information about TWI's capabilities please contact us.