Forging, rolling, extrusion and related metal forming processes are key industrial technologies required to produce many highly reliable parts economically. Due to competition at the low-cost production end of the market many steel manufacturers are being driven towards the production of technologically demanding steel types and the process control accuracy demanded by higher integrity, higher value components. Knowledge of microstructure development during processing is an essential input parameter for process models capable of bringing cost benefits via reduced energy consumption reduced harmful emissions and less material wastage.

A typical hardness map (left) of the Waspalloy is compared with
the numerical model prediction of variation in strain (right) 

To underpin suitable process models, NPL, in partnership with the Universities of Sheffield and the University of Wales (Swansea) is developing good practice for microstructural examination in dynamic high temperature regimes. This includes defining local deformation conditions for comparison of computed and measured maps of microstructural evolution: information necessary for validation of models that include a consideration of the strain history of the testpiece.

Measurements of the spatial variation in mechanical properties are being made at NPL, using the SIMM (Scanning Indentation Mechanical Microprobe). The SIMM uses the principle of depth sensing hardness. The concept is a little like a nanoindenter, dynamically measuring force and penetration depth, but it works more quickly and to higher loads. The measurement process is fully automated to map large or small areas or conduct line scans. Force and displacement are monitored continuously during the complete indentation cycle. The machine operates in the micro to mesoscale range, 0.05-20N, with a load resolution of 2 mN and a depth sensing resolution of 20 nm using a specially designed capacitance displacement sensor.

The resulting measurements are compared to measurements of microstructural variation and also compared with the outputs of numerical models, run by Sheffield and Swansea, of hot deformed compression testpieces.