Cracked D2 punch after a one-shot harden and single temper
An HCHCr (AISI D2-type) punch, 40 mm diameter with a 0.5 mm fillet at the shank-to-body junction, is hardened in the tool room. The trainee loads it cold into an open gas-fired furnace set at 980 °C, holds for 15 minutes, quenches in still oil at 30 °C, and tempers once at 180 °C for one hour. Measured hardness is 62 HRC. It produces about 150 blanks from 2 mm mild steel, then a crack opens at the fillet and the cutting edge shows pitting and a softened band. The matching die block from the same bar stock has chipped along its edge. There is no preheat step, no protective atmosphere or packing, and no stress-relieving of the machined blank. The supervisor is preparing a complaint to the steel supplier. You are asked for a technical opinion.
1. Analyse the likely causes of the fillet crack and the softened, pitted edge. Is the steel supplier at fault?
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At least five process faults are visible, so the steel is unlikely to be the cause:
- No preheat. A 40 mm D2 section put cold into a 980 °C furnace develops a steep thermal gradient. It must go to 600-650 °C in one or two stages, equalise, then be raised to austenitising temperature.
- Sharp geometry. R0.5 at a section change is a stress raiser under quench stress. Open it to R1.5-2 mm and polish the transition.
- No atmosphere protection. An open flame gives scaling and decarburisation; the pitted, softened band is classic decarburised surface. Use controlled atmosphere, vacuum, or packing in cast-iron chips.
- Single low temper. One hour at 180 °C leaves untempered martensite and retained austenite. Temper twice at 200-250 °C, one hour per 25 mm of section, cooling to room temperature between tempers.
- No stress relief of the machined blank before hardening.
Also, still oil at 30 °C is on the cold side for D2; 50-80 °C with agitation gives better through-hardening and less distortion.
2. Set out the corrected hardening sequence and state what hardness you would expect and why.
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Rough machine, then stress relieve at 650-680 °C with a furnace cool. Finish machine leaving grinding allowance. Preheat to 600-650 °C in one or two stages, then austenitise at 980-1010 °C with a soak of 30-45 minutes at temperature for a 40 mm section. Quench in agitated oil at 50-80 °C, and temper immediately while the part is still above 50 °C.
Temper twice at 200-250 °C. Expected hardness is about 58-61 HRC. The 62 HRC reading was taken on an under-tempered surface and is not the property wanted in a blanking punch: at that level the edge is hard but brittle, which is exactly what chipping and fillet cracking show. For a punch, toughness with 58-60 HRC is the correct compromise. Finish grind after hardening to remove decarburised skin and hold the cutting edge, and verify with at least three HRC readings on the ground surface plus a hardness traverse if the edge is critical.
3. How would you prove whether decarburisation occurred, and what depth is acceptable on a cutting edge?
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Take a coupon from the same bar, harden it with the corrected cycle, and examine it: mount and polish a cross-section, etch in 2% nital, and look for a free-ferrite layer at the surface under the microscope. Measure its depth on the etched section. Back it up with a microhardness traverse from the surface to the core at 0.1 mm steps; a soft skin of more than about 20-30 HV below core hardness confirms decarburisation.
On a cutting edge, no decarburised layer is acceptable in service because it is soft and will wear or spall within a few hundred strokes. Any layer must be removed by grinding, which is why a grinding allowance of at least 0.3-0.5 mm per face is left before hardening. Also check that the quench did not leave a soft spot by hardness testing at several points along the edge.