Microstructures before and after grinding. Heat generated during
grinding re-Austenized the surface layer to a very high temp (>1200C) and
resulted in "giant prior Austenite grains---5~6 grains in photo 4.jpg,
and > 100 grains in "before". You can also see carbide particles in "before"
are gone and replaced with R.A. in photo 4.jpg.
This forging die was said to fracture at only a few hits.
A few means two or three in numbers. Very coarse Martensite + high % of
retained Austenite should be the reason for its short service life.
It was said to be Austenized at 1020C for 40 min, quenched, double tempered at 540 x 2 hr.
I took it with a grain of salt.
Fractured surfaces of a hot forging tooling made of H13 and
gas nitrided. The tooling broke into 5 pieces only after a few hits.
Holes should be chamfered to avoid stress concentration.
Grain boundary carbides before / after Q/T.
DF1: annealed condition, Dark field illumination.
The rest: Q / T condition, Dark (2~4 )and bright field (5) illumination.
This is another tooling that broke into pieces.
Chamferring* before nitriding helps only a little because the tooling still cracked.
Grinding corners after nitriding helps more in preventing cracking. *:Removing the brittle white compound layer at corners.
Networks of nitride and carbide are found on grainboundary, as
identified by the EDS results. Nitride and carbide are ceramic in nature.
They separate easily from matrix and result in microcracks.
Micrographes are taken from "diffusion layer".
Lots of microcracks are seen.This proves the fact that
nitriding increases wear resistance at the expense of toughness.
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IP卡
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发表于 2011-7-15 11:07:34
