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Abstract
Hardenability, tensile, and impact properties have been determined for two nickel-molybdenum powder-formed steels. The compositions of the two steels were Fe-1·8% Ni−0·5% Mo and Fe −0·5% Ni −0·6% Mo; carbon contents studied for both materials were 0·2, 0·3, and 0·4%. Preforms (50 × 50 × 127 mm) were pressed and then sintered for 1 h in a dissociated ammonia atmosphere at 1120°C, to provide a density of 6·5 g/cm3. Preforms were hot formed to full density by upsetting (plane strain) at 980°C or by re-pressing (uniaxial strain) at 1040°C.
Standard 25 mm-dia. × 100 mm-long Jominy end-quenched tests were made to determine hardenability. Room-temperature tensile properties were determined using 6·4 mm-dia. specimens with a 25 mm gauge-length. Standard Charpy V-notch specimens were used to determine the impact properties at −40, 0, 22, and 100°C. Triplicate tests were made in the as formed as well as the quenched-and-tempered conditions.
The additional flow that was realized by upsetting provided higher tensile and impact strengths than re-pressing. Tensile ductility was the same for both processing conditions. Tensile yield strength (heat-treated to ∼26 HV) increased with increasing carbon content for the slack quenched material and levelled off for fully martensitic structures. Room-temperature impact strength also increased with increasing carbon content owing to a decrease in the amount of ferrite in the slack quenched structure. Increasing carbon content caused a decrease in impact strength once fully martensitic structures were achieved.
Impact strength at −40°C ranged from ∼20 to 40 J. Room-temperature impact strength ranged from ∼20 to 50 J. These properties were obtained for materials heat-treated to a UTS level of ∼850 N/mm2. At the lower carbon contents the hardenability curve was found to fall within the AISI ‘H’ band for 4620 wrought steel. At the higher carbon level (0.4% C) the Jominy curves were close to the lower limit of the ‘H’ band for the 4640 wrought steels.
Notes
* Manuscript received 4 September 1973. Contribution to a Symposium on ‘PM Alloys and Properties’, held in Eastbourne on 19-21 November 1973.