Grinding cracks caused by failure to temper a part - Machining defects

Grinding cracks caused by failure to temper a part - Machining defects - Fractography

Figure 1: (a) Two dies made from AISI D2 tool steel that cracked after finish grinding (cracks accentuated with magnetic particles), (b) Macroetching (10% aqueous nitric acid) of the end faces revealed grinding scorch. These dies were not tempered after hardening.

Defect name: Grinding cracks
Record No.: 2329
Type of defect (Internal/Surface): Surface
Defect classification: Machining defects, fractography
Steel name: AISI D2 tool steel
Steel composition in weight %: No data.
Note: Failures due to finish grinding. Cracking due to the stresses and microstructural alterations caused by grinding is a relatively common problem. In many cases, the grinding technique is not at fault, because the microstructure of the part rendered it sensitive to grinding damage due to failure to temper the part or because the part was overaustenitized and contained substantial unstable retained austenite.
Figure 10 shows an example of grinding cracks due to failure to temper the part. Two AISI D2 tool steel dies, which measured 57 x 60 x 29 mm or 51 mm thick (2'A x 23/8 x l'/8 in. or 2 in. thick), were observed to be cracked after finish grinding. The cracks are emphasized with magnetic particles. Macroetching of the surfaces revealed the classic scorch pattern indicative of abusive grinding. The failure, however, was not due to poor grinding practice, but was caused by failure to temper the dies. The interior hardness was 63 to 64 HRC, typical for as-quenched D2. The scorched surface was back tempered to 55 to 58 HRC. It is difficult to grind as-quenched high-hardness tool steels without damaging the surface.
Reference: Not shown in this demonstration version.