The opening presentation came from Ulf Engstrom, Hoganas AB, on behalf of his co-authors, Caroline Larsson, Sigurd Berg and Christophe Szabo. It is recognised that “green” issues are becoming an increasingly important factor in determining the relative merits of competing materials and technologies.
The benefits of PM, as compared with wrought steel processing routes in terms of reduced energy consumption and higher material utilisation in the component forming process itself, are well recognised. However, the comparisons provided at the opening of this presentation were considerably more rigorous than this in that they involved full life cycle analyses, beginning with mining/extraction of raw materials and ending with end-of-life recycling of components.
These analyses considered the environmental impact factors:
- Global Warming Potential (CO2, CH4) – GWP
- Photochemical Ozone Creation Potential – POCP
- Acidification Potential (SO2, NOx) – AP
- Eutrophication Potential (NOx, PO4) – EP
As a case study, production routes for an injection yoke by wrought steel processing and by PM, using two different PM materials (Astaloy CrM + 0.3%C and Distaloy AE + 0.5%C), were assessed (Fig. 1).
Firstly, the presented results of these analyses showed the very significant advantage of PM over the wrought steel route in relation to the environmental impact factors (Fig. 2).
Secondly, the comparisons of the two PM material types showed the environmental credentials of using the chromium-containing Astaloy CrM material as compared with the 4% Ni Distaloy AE. Although the move away from PM materials highly alloyed with Ni is partially motivated by cost considerations, these “green” issues are also of increasing importance in material selection and Cr- and leaner Ni-alloyed grades offer the opportunity to fulfil these requirements.
The leaner Ni-alloyed material highlighted was Hoganas AB’s Distaloy AQ, containing diffusion-alloyed additions of 0.5%Ni and 0.5%Mo. This material can achieve density levels over 7.3 g/cm3 when warm compacted and can achieve very high levels of dynamic properties in the heat treated condition, particularly after case hardening.
Fig. 3 shows the levels of plane bend fatigue strength attainable in the case-hardened and through-hardened conditions at room temperature compaction and warm compaction density levels. The benefits of warm compaction in relation to fatigue strength are related to the reduction in both the size and frequency of occurrence of large pores in the sintered structure (Fig. 4)
The highlighted Cr-alloyed materials were the fully pre-alloyed grades, Astaloy CrA (1.8% Cr) and Astaloy CrM (3% Cr, 0.5%Mo). In the assessments presented, Astaloy CrA was used as a base for the elemental addition of either 1% Cu or 2% Ni. Warm compaction of the material with the 2% Ni addition showed that a density level of around 7.25 g/cm3 could be attained compared with around 7.1 g/cm3 for room temperature compaction.
These materials are well suited to processing by sinter-hardening and Fig. 5 shows that, on sinter-hardening with a carbon gradient, the Astaloy CrA + 2% Ni material can achieve fatigue and tensile strength levels comparable to much more highly alloyed heat treated materials and unique combinations of fatigue strength and elongation, even at room temperature compaction density levels.
The presentation concluded with some predictions as to how these material and process developments can enhance PM’s capability for attacking growth opportunities in challenging product applications. Fig. 6 provides some views as to how the highlighted materials might fit into a number of important existing and potential product niches, including the industry’s “holy grail” of transmission gears.
Finally, the promise was discussed of future developments of “low pressure processing”, involving vacuum sintering and gas quenching, that might provide further enhancements in the properties of the highlighted Cr-alloyed grades and how these processes might be combined with state-of-the-art hard finishing operations to provide an optimised PM gear production route.
Dr David Whittaker is a consultant to the Powder Metallurgy and associated industries. Contact +44 1902 338498 email: [email protected]
PowderMet2013 was organised by the Metal Powder Industries Federation. For more information please visit the MPIF website: www.mpif.org