The potential of PM Mn steels has been established yield, tensile and bend strengths of 499, 637 and in laboratory experiments. This paper deals with 1280 MPa, respectively, with impact energy of 18 J, and sintering of Fe-(2-4)Mn-(0•3/0•7)C, also with 0•85%Mo tensile and bend strains of 1•17 and 1•57%, were achieved addition, in an industrial pusher furnace at 1180°C in an for the Fe-2Mn-0•85Mo-0•5C alloy, marginally superior atmosphere of 25% hydrogen plus 75% nitrogen, to Fe-2Mn-0•7C. For the sinter hardened Fe-4Mn-0•3C obtained from a cryogenic liquid, giving an inlet dew-alloy yield, tensile and bend strengths were 570, 664 and point of −55°C. Tensile, bend (including fatigue) and 1263 MPa, respectively, at an acceptable impact energy miniature Charpy specimens were sintered in flowing of 14 J, with tensile and bend strains of 0•52% and 1•8%. gases and in semiclosed containers with a getter of ferro-Many of the results compare favourably with the requiremanganese, carbon and alumina. The quenched and tem-ments of MPIF standard 35. Mn is a more effective pered state was investigated, as was sinter hardening strengthening agent than either Ni or Cu, or their combi-(cooling rate of 55 K min−1), simulated for comparison nation, though generally at reduced plasticity. with slow cooling at 10 K min−1. As there was no forma-PM/1086 tion of oxide networks at the combination of sintering temperature and dewpoint, in accordance with the Dr Dudrova ´(

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Nucleation of microcracks, their growth and coalescence are analysed in powder metallurgy (PM). Fe-1 . 5Cr-0 . 2Mo-0 . 7C steel by fractography allied to surface replica microscopy -at several stress levels as the maximum tensile stress in three-point bend specimens was raised to 99 . 6% of the transverse rupture strength TRS of 1397 MPa. The fatigue limit in this material is y240 MPa, at which stress level no microcracks were detected in static loading. Numerous microcracks, ranging in size from ,5 to y20 mm, however, were nucleated above ,800 MPa, i.e. beyond the yield strength of ,620 MPa. With increasing stress, some microcracks became dormant, whilst others grew subcritically, stress step-wise, to some 400 mm. Of particular importance are observations of the coalescence of two and three of such microcrack systems to produce a critical, propagating crack. The then estimated stress intensity factor K a , could reach K 1C , independently estimated to be ,36 MPa m 1/2 . Microcrack coalescence was associated with easy paths for crack growth, principally prior particle boundaries linking pores. Ways of making subcritical crack growth more difficult and hence improving both static and dynamic mechanical properties, are considered.

A B S T R A C T Detailed quantitative micrographic data are presented for Stages I and II of a PowderMetallurgy Fe-1.5Cr-0.2Mo-0.7C steel specimen fatigued in bending with R = −1 at 24 Hz and a stress amplitude of 312 MPa. The fatigue limit was ∼240 MPa, at which stress level no microcracks were detected in static loading. Testing was interrupted at 100 cycles and at further 29 intervals until failure after 49 900 cycles. For each arrest, surface replicas were made in the two regions where maximum stress was applied. Microcracks could nucleate below 100 cycles, when their sizes ranged from <5 to ∼20 μm. Fractographic examination identified the failure-originating site, which was then associated with the crack system observed on the 'last' pre-failure micrograph. Detailed examination of the eventual failure region showed nucleation, at various cycle intervals, of 18 microcracks, their subcritical growths, arrests and coalescences with continuing cycling to form a critical crack 2.25 mm deep. Stepwise microcrack growth was probably rapid -to the next arrest or coalescence. For each (micro)crack size stress intensity factors, K a s, were estimated and, at the end of Stage II, for the coalesced crack, K a reached K 1C , independently estimated to be ∼36 MPa m 1/2 . a = semi-minor axis of a semi-elliptical crack (crack depth) c = semi-major axis of a semi-elliptical crack K = stress intensity factor K 1C = critical (plane strain) stress intensity factor-fracture toughness N = number of cycles R = stress ratio R m = (ultimate) tensile strength R p0.2 = 0.2% offset yield strength TRS = transverse rupture strength (calculated assuming elastic deformation) S = stress amplitude I N T R O D U C T I O NFatigue studies of structural Powder Metallurgy, PM, materials generally have followed the approach adopted for wrought materials, but some important differences need noting. Most PM steels contain porosities above 7% and, not only have the modulus lower than that for their wrought counterparts, therefore also have fatigue resistance and toughness. Much of the work has been carried Correspondence: A. S. Wronski. E-mail: a.wronski@bradford.ac.uk out on smooth specimens, generating S-N curves, supplemented by fractographic examinations, including reports of fatigue striations. 1,2 The strength of PM materials may not be much lower than that of their wrought counterparts, but because the fracture toughness of PM steels is lower, typically 20-35 MPa m 1/2 , they have smaller critical crack sizes. Literature data exist on the roles of (especially interconnected) porosity, 1,3 the surface 1,3-5 and prior particle boundaries 4,6 in the failure mechanisms, which have been modelled. 4,7 Fatigue cracks at several surfaces sites 8 214