Premature tempering accounted for the cracking during treatment of this 21-in.-OD ring die - Tool steels - Fractography

Premature tempering accounted for the cracking during treatment of this 21-in.-OD ring die - Tool steels - Fractography - Heat treatment defects

Figure 1: Premature tempering accounted for the cracking during treatment of this 21-in.-OD ring die, a portion of which is illustrated. Made of high-carbon, high-chromium tool steel, it was apparently much hotter than 200 F when the air-quench was halted. The die was put into the tempering furnace before the hardening transformation had progressed appreciably. In this grade of steel, such transformation does not even begin until it has cooled to about 400 F. The temperature reached in tempering, about 925 F, does not transform this steel readily, so it was still unhardened when the tempering operation was completed. Cooling in air following removal from the tempering furnace again started the hardening transformation, but the die was then water-cooled—a much too drastic procedure. The intent wasto cool it so that it could be handled for hardness testing. Stresses were set up and cracking resulted.
If this die had not been water-cooled following the temper, but had been allowed to cool to room temperature in air, and had then been re-tempered, it is probable that no trouble would have occurred.
The original error, of course, was removal of the die from the quench too soon. The second error was cooling in water. A double temper would have corrected the trouble caused by the original error. Microscopic examination furnished the clue which brought to light the faulty procedure. Note the microstructures on Fig. 2.

Figure 2: Left: Normal microstructure (7O0x magnification) of hardened-and-tempered high-carbon, high-chromium steel showing carbides in a tempered martensitic matrix. Right: Structure of ring die shown on Fig. 1. This structure showing carbides in untempered martcnsitic matrix, is typical of as-quenched steel of this grade and indicates that the die did not respond to tempering because of the incomplete quench.

Defect name: Cracks
Record No.: 2899
Type of defect (Internal/Surface): Internal, surface
Defect classification: Fractography, heat treatment defects
Steel name: Tool steel
Steel composition in weight %: No data.
Note: No data.
Reference: Not shown in this demonstration version.