The heat treatment process for common steels: Heating and cooling specifications for various heat tr

1. Quenching
(1) Carbon and alloy structural steels
① For carbon steel parts with complex shapes and some alloy steel parts, water, and oil are used for cooling.
② For carbon steel parts with complex shapes and some alloy steel parts, this section is used.
(2) Chromium stainless steel and austenitic steel
(3) Tool steel and low alloy steel Preheating depends on the shape and size of the workpiece.

2. Normalizing and annealing
(1) Normalizing
(2) Annealing

3. Tempering, aging and stress relief
(1) Tempering Note: Tempering time is calculated according to this coefficient below 530℃.
(2) Aging (austenitic steel)
(3) Stress relief
① For parts with simple shapes, air cooling is used to remove straightening stress.
② When the actual heating rate of the furnace exceeds ≤80℃/hour and cannot meet the process requirements, the parts are heated to 300℃ and kept warm for 1 to 2 hours, and then heated to 100℃ and kept warm for 1 hour, and then kept warm for 1 hour for each 100℃ increase until the holding temperature is reached. For parts with complex shapes and easy to deform, such as valve stems, the temperature must be guaranteed to be ≤50℃/hour and manual control can be used when necessary.
A few explanations of the process specifications:
(1) "Through" refers to the time from the furnace reaching the temperature after the workpiece is loaded into the furnace to the time when the workpiece and the furnace fire color are consistent.
(2) "Unlimited" in the furnace loading temperature column generally refers to temperatures below the holding temperature in the curve. (If there are two holding temperatures in the curve, the holding temperature of the first section should not exceed)
(3) The quenching holding time of 38CrMoAlA steel should be extended by 30% as required.
(4) The tempering holding time of hardened parts is required to be at least 1 hour.
(5) Stress removal time: The time for stress removal from machining and straightening shall not be less than 3 hours; the time for stress removal from welding and casting shall not be less than 4 hours.
(6) When stress removal from straightening is performed after nitriding, nitrogen shall be passed through the entire stress removal process.
(7) According to Z75.22-86 GH2136, the aging heat treatment is ≤300℃ before entering the furnace, the temperature is raised to 710±10℃ with the furnace, and the heat preservation is carried out for 12~16 hours. After exiting the furnace, the temperature reaches HRC32~42.

4. Chemical heat treatment process specifications
(1) Nitriding
① For specific provisions of furnace cooling, see the precautions.
② The insulation time of the nozzle group is 6~8 hours.
③ Add pills to remove the passivation film on the surface of the parts, and the amount is 100~120 pills per furnace.
Precautions:
① Before nitriding, the surface rust of the parts must be removed, and the surface oil stains must be cleaned with gasoline.
② When loading parts into the furnace, all surfaces to be nitrided must not touch each other. Pay special attention not to place large workpieces near thermocouples. It is strictly forbidden to place workpieces against thermocouples. For partially nitrided parts, check whether the tinned parts are correct. For screw holes below M16 and blind holes and through holes below ￠16, threaded parts, and back-cut grooves of nitrided parts with threads protected by tin plating, paint protection should be applied before nitriding.
③ After the nitrided parts are put into the furnace, nitrogen should be passed through for 30 minutes to remove the air in the box. The ammonia pressure should be controlled at 100-150mm oil column. After the air is removed, the pressure should be adjusted according to the process requirements to control the decomposition rate. Ammonia should not be interrupted from beginning to end during the nitriding process.
④ When nitriding is cooled, turn off the power first stop the ammonia when the furnace cools to ≤150℃, and take out the parts.
⑤ The ammonia pressure in the furnace is for reference only. The decomposition rate should be controlled during operation.

(2) Carburizing
1) Solid carburizing: The holding time depends on the number of boxes in the carburizing furnace and the size of the box.
2) Gas carburizing
A few points:
①Solid carburizing agent formula: 7%BaCO3+2.5%Na2CO3, the rest is 90.5% charcoal, and the ratio of new and old carburizing agents is 1:2.
②Gas carburizing agent:
(a) Benzene drip rate: control at 30-35 drops/min when heating, and control at 60-70 drops/min when keeping warm.
(b)* Kerosene + alcohol 50% each, the drip rate is the same as benzene. When using it for the first time, conduct a test to determine the appropriate drip-rate.
③Look at the predicted sample 1-1.5 hours before the carburizing insulation time arrives, and determine the furnace discharge time based on the measured results.
④When loading the furnace for gas carburizing, keep a certain distance between the parts, at least 5-10mm. The distance between the parts or the box wall of the solid carburizing box shall not be less than 15-20mm.
⑤If network carbides are found in the carburized layer after carburizing, normalizing treatment should be carried out to eliminate them.

5. Specifications for the heat treatment process of forging dies
Quenching of forging dies and tire dies: immediately put them into the prepared tempering furnace after oil discharge.
Tempering: 5CrNiMo tempering oil cooling.
Note:
(1) The dovetail adopts self-tempering. Quenching process: small molds (H≤250mm) are all immersed in oil for 5′～8′ (medium and large molds are immersed in oil for 8′～12′) and then the dovetail is lifted out of the oil surface. When the oil surface burns, it is immersed in oil again. Repeat 2 to 3 times to allow the dovetail to self-temper. Medium-sized molds H=250～400mm, large molds H>400mm.
(2) The oil outlet temperature is 250～300℃, not lower than 250℃, and a tempering furnace of 350～400℃ is prepared. After oil discharge, it is immediately put into the tempering furnace.

Precautions for heat treatment of forging dies:
(1) Molds must be annealed before repair or renovation to prevent cracking.
(2) The forging die must be protected during mold quenching and heating. See the schematic diagram for protection. The working surface of other molds should be protected by pig iron filings and dry charcoal to prevent decarburization, and the mold should be padded 50-100mm.
(3) Tempering should be carried out in time after quenching.
(4) The holding time in the curve = mold height × heating coefficient (min/mm). "Through" refers to the duration from the furnace temperature rising to the color of each part of the workpiece being consistent with the furnace color.