We need to properly select optimal chemical composition to obtain a given organization for cast iron production,heat treatment is also an important way to further adjust and improve matrix structure in order to improve the performance.Cast iron heat treatment has some way with cast steel heat treatment,but also exist differences.Cast iron heat treatment generally could not improve the morphology and distribution of graphite in the original organization.For grey cast iron, the stress concentration caused by flake graphite plays a leading role on cast iron properties, so strengthen effect on heat treatment of grey cast iron is far less significant than steel and ductile iron.Thus heat treatment process for gray iron mainly is normalizing,annealing,ect.For ductile iron,as the graphite is a ball shape,which will greatly reduce fragmentation function of base organization.By heat treatment,it could give full play role,which can significantly improve the mechanical properties of the ball.So ductile iron is like steel,the heat treatment process includes annealing, normalizing, tempering, quenching temperature, induction heating quenching and surface chemical heat treatment, etc.
Cast Iron Heat Treatment Process
1.Stress relief annealing
Because the casting wall thickness is not uniform, in the heating, cooling and the phase change process, force and organizational stress can produce effect force and organizational stress.Besides,as internal of large parts after machining easily produces residual stress and we have to eliminate all the internal stress.Normally heating temperature of stress relief annealing is 500 ~ 550 ℃ and heat preservation time is 2~8 hours,then the furnace cooling(gray iron) or air cooling (ductile iron).Using this new process can eliminate stress 90 ~ 95% in the casting, but cast iron organization will not change.If the temperature over 550 ℃ or holding time is too long,it will cause the graphitization,reduce casting strength and hardness.
2.High temperature graphitizing annealing
When casting cooling,surface and thin section,tend to produce white.White tissue is hard and brittle,poor processing performance, easy peeling off.So you must adopt the method of annealing or normalizing eliminate white tissue.Annealing:heating to 550-950 ℃ heat preservation 2 ~ 5 h,then the furnace cooling to 500-550 ℃ and then air cooling.
3.Normalizing of ductile iron
The purpose for ductile iron normalizing is to pearlite matrix organization, and refine the grain size,smooth texture,in order to improve the mechanical properties of the castings.Sometimes normalizing is also ductile iron surface quenching in organizational preparation, normalizing is divided into high temperature normalizing and low temperature normalizing.
4.Quenching and tempering of ductile iron
In order to improve the mechanical properties of ductile iron, general casting heated to Afc1 above 30 ~ 50 ℃ (Afc1 represents A form at the end of the temperature when the heating),quenching in oil after heat preservation, then get martensite structure.
5.Multiple temperature quenching of ductile iron
In order to improve some castings' surface hardness,wear resistance and fatigue strength,surface quenching can be used.Gray cast iron and ductile iron castings could also adopt surface hardening.Generally use high (in the) frequency induction heating surface quenching and electrical contact surface quenching.
7.Chemical Heat Treatment
For surface abrasion or oxidation resistance,corrosion resistance of investment castings,can use a similar chemical heat treatment technology of the steel.