Toolform Case study:

Variability in the Performance of Nitrided Forging Dies

Stokes Forgings has forge presses ranging from 500 to 2500 tonne capacity producing components such as automotive axle parts (Photos courtesy Stokes Forgings)

TWI has investigated the variability of performance in nitrided forging dies for a West Midlands based forging company.

Stokes Forgings, located in Walsall, West Midlands, is a producer of components for use in automotive and powertrain applications. The Stokes Forgings Group is the largest supplier in the UK of medium to large volume ferrous forgings in the range 0.2kg to 35kg.

Die life is of utmost concern to companies involved with metal forging and forming. For closed die forgings, these must withstand repeated impacts from hammer or press loads under thermal cycling, whilst being robust enough to be able to stamp out the final shape. The majority of die damage to the dies is caused simply by wear at the die corners and other stress concentrating areas.

There is variability in the durability of pre-hardened, nitrided M2 forging tools. Typical examples of the problem occur on the mold and finish tools for the pressing of automotive components. Stokes Forgings wished to investigate this issue with a view to imposing stricter quality control measures at their nitriding sub-contractors and thus remove the variability in nitrided layer performance. Stokes Forgings provided two forging dies, representative of the variation in forging die lifetimes, to TWI for analysis.

The nitriding of the forging dies was performed using a fluidised bed process that uses alumina (Al2O3) combined with appropriate gases. TWI analysed the microstructure, microhardness and surface composition of the dies and demonstrated that there is a strong correlation between the nitrided layer depth and the tool life. The thicker the nitrided layer the longer the life of the die. Temperature variations of only a few degrees and total nitriding time had an effect on depth of nitrided layer achieved and the hardness.

Nitrided surface layer before use shows smooth surface	After forging, nitrided surface cracked along grain boundaries

For further information on this work, please contact Melissa Riley at TWI (email: melissa.riley@twi.co.uk).

This support, provided to Stokes Forgings, was made possible through the Toolform Strand of the Advanced Metals Technology Initiative (AMTI) funded by the DTI, Yorkshire Forward and the European Union through the South Yorkshire Objective 1 programme. The AMTI project is managed by NAMTEC, the National Metals Technology Centre.

For more information on the Toolform programme, please contact: Eliana Fu (email eliana.fu@twi.co.uk.