What is annealing? Purpose and technology
Annealing is a heat treatment of metal that slowly heats the metal to a certain temperature for enough time and then cools at an appropriate rate. Annealing heat treatment is divided into complete annealing, incomplete annealing and stress relieving annealing. The mechanical properties of annealing materials can be tested by tensile test or hardness test. Many steel products are supplied under annealing and heat treatment. For steel hardness testing, rockwell hardness tester can be used to test HRB hardness. For relatively thin steel plate, steel belt and thin-wall steel pipe, rockwell hardness tester can be used to test HRT hardness.
The purpose of annealing is:
To improve or eliminate all kinds of microstructure defects and residual stress caused by steel in the process of casting, forging, rolling and welding to prevent the workpiece from deformation and cracking.
To soften the workpiece for machining.
Refine the grains and improve the structure to improve the mechanical properties of the workpiece.
Prepare tissue for final heat treatment (quenching, tempering).
Common annealing processes are:
Fully annealed. Thick superheat microstructure of medium and low carbon steel after casting, forging and welding. All into the workpiece heated to ferrite austenite temperature more than 30 ~ 50 ℃, the heat preservation for a period of time, and then with the slow cooling furnace, austenite during cooling to change again, can make the organization of the steel.
Ball annealing. Used to reduce the high hardness of tool steel and bearing steel after forging. The workpiece heated to begin to form austenitic steel temperature above 20 ~ 40 ℃, slow cooling after heat preservation, in the process of cooling of pearlite lamellar cementite into a ball, reducing the hardness.
D. Isothermal annealing. To reduce the high hardness of some alloy structural steels with high content of nickel and chromium for machining. Generally, the austenite is cooled to the most unstable temperature of the austenite at a relatively fast speed. If the temperature is kept at a proper time, the austenite is transformed into tortoids or sorbite, and the hardness can be reduced.
E recrystallization annealing. It is used to eliminate the hardening phenomenon (increase in hardness and decrease in plasticity) of wire and sheet during cold drawing and cold rolling. Heating temperature is commonly steel began to form the austenitic temperature below 50 ~ 150 ℃, only in this way can eliminate the strain hardening effect to soften a metal.
Graphite annealing. It is used to turn cast iron containing large amounts of cementite into forgeable iron with good plasticity. Casting process is be heated to 950 ℃ or so, appropriate cooling after heat preservation time, make cementite decomposition form flocculent mass graphite.
D diffusion annealing. It is used to homogenize the chemical components of alloy castings and improve their performance. The method is to heat the casting to the highest temperature possible without melting, and keep it for a long time, until the various elements in the alloy diffuse to a uniform distribution and then cool down.
To remove stress annealing. To eliminate the internal stress of steel castings and welding parts. For began after the formation of austenitic steel heating temperature under 100 ~ 200 ℃, holding in the air after cooling, can eliminate internal stress.