The Tempering Process Of Nodular Cast Iron

Heat treatment process: overall quenching

Quenching: heating at 875~925ºC temperature, holding for 2~4h, quenching into oil to obtain martensite matrix structure

Tempering: high temperature tempering at 550~600ºC to obtain tempered sorbite structure + a small amount of ferrite + spheroidal graphite

350~500ºC tempering at medium temperature to obtain tempered troostite structure + a small amount of ferrite + spheroidal graphite

Tempered at 150~250ºC to obtain tempered martensite structure + a small amount of ferrite + spherical stone cui

Others: Purpose: to obtain high strength (>700MPa), low elongation performance

Note:

• ①It can prevent cracking and can be quenched into 200ºC hot oil;
• ②It is best to normalize before quenching

Heat treatment process: surface and chemical heat treatment

Quenching: flame or induction heating will heat the surface of the casting to 850~950ºC, the heating depth can reach 2~4mm, water quenching

Tempering: 380~410ºC tempering furnace tempering

Others: Purpose: improve surface hardness and wear resistance, and obtain a martensite layer with a hardness of 600~700HV

Note: Use normalizing treatment before quenching, pearlite>70% (volume fraction)

Quenching: Nitriding at 550~600ºC, depth of 0.1m

Heat treatment process: Austempering

Quenching: high temperature isothermal quenching: heating to 875~925ºC, holding for 2~4h, quenching in 400~450ºC salt bath for 1~6h, then cooling to room temperature

Others: The main thing obtained is the structure of upper bainite + more retained austenite. Cast iron can have high toughness, high strength and medium hardness, and has good work hardening ability. Its performance is high elongation and high fracture toughness, which should be attributed to retained austenite

Quenching: Low temperature austempering: the heating process is the same as above. Quench in a salt bath at 235~350ºC for 1~6h, then cool to room temperature

Others: The main obtained is lower bainite + part of retained austenite. It has high strength, certain plasticity and toughness, and has the ability to work hardening. It has much higher wear resistance than other ductile cast irons.

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