On 2018-08-08 14:48:54
heat treatment for screw products
What are the methods and means of heat treatment? How are they implemented? Today, I would like to give you a brief introduction of this aspect, hoping to help you.
1、 Heat treatment method: different heat treatment methods can be selected according to different purposes and purposes.
Quenched and tempered steel: high temperature tempering after quenching (500-650 ℃)
Spring steel: medium temperature tempering after quenching (420-520 ℃)
Carburizing steel: quenching after carburizing, low temperature tempering (150-250 ℃)
The general rule of low carbon steel and medium carbon (alloy) steel after quenching into martensite is that with the increase of tempering temperature, the strength decreases, and the plasticity and toughness increase. However, due to the different carbon content of low carbon steel and medium carbon steel, tempering temperature has different effects on them. Therefore, in order to obtain good comprehensive mechanical properties, the following methods can be used respectively:
(1) Low carbon martensite can be obtained by selecting low carbon (alloy) steel and tempering at low temperature below 250 ℃ after quenching. In order to improve the surface wear resistance of this kind of steel, only by increasing the carbon content of each surface layer can the surface carburization be carried out, commonly known as carburized structural steel.
(2) After quenching (the so-called quality adjustment treatment), medium carbon steel with high carbon content shall be used and tempered at high temperature (500-650 ℃), so as to maintain sufficient strength under high plasticity conditions. Usually, this kind of steel is called quenched and tempered steel. If you want to obtain high strength and prefer to reduce plasticity and toughness, you can use low-temperature tempering for gold containing tempering with low carbon content, so as to obtain the so-called "ultra-high strength steel".
(3) For steel grades with carbon content between medium and high carbon (such as 60, 70 steel) and some high carbon steel (such as 80, 90 steel), if it is used to make springs, in order to ensure high elastic limit, yield limit and fatigue limit, medium temperature tempering is used after quenching.
2、 Operation process:
(1) Quenched and tempered steel:
1. Preheating treatment: normalizing - > annealing (pearlitic steel) - > high temperature tempering (martensitic steel)
(1) The purpose of normalizing is to refine the grains, reduce the banded degree in the structure, adjust the hardness and facilitate machining. After normalizing, the steel has fine equiaxed grains.
2. Quenching: heat the steel body to 850 ℃ for quenching. The quenching medium can be selected according to the size of the steel piece and the hardenability of the steel. In general, water, oil or even air can be selected for quenching. The steel in quenching state has low plasticity and high internal stress.
(1) In order to make the steel have high plasticity, toughness and proper strength, the steel should be tempered at a high temperature of about 400-500 ℃. The steel which is more sensitive to temper embrittlement should be cooled rapidly after tempering to prevent the occurrence of temper embrittlement.
(2) If the parts are required to have a particularly high strength, they are tempered at about 200 ℃ to obtain a medium carbon tempered martensitic structure.
(2) Spring steel:
1. Quenching: 830-870 ℃ oil quenching.
2. Tempering: tempering at 420-520 ℃ to obtain tempered troostite structure.
(3) Carburizing steel:
1. Carburizing: a chemical heat treatment refers to the infiltration of element C into the surface of steel parts in the active medium containing certain chemical elements at a certain temperature. Partial preheating (850 ℃) carburizing (890 ℃) diffusion (840 ℃) process
2. Quenching: carbon and low-alloy carburized steel are usually directly or once quenched.
3. Tempering: low temperature tempering to eliminate internal stress and improve the strength and toughness of carburized layer. The tempering temperature of the middle tooth thread produced by our company is about 360 ℃, the tempering temperature of the self tapping screw (panel nail) is about 200 ℃, and then it is cooled to 34-35 ℃ and 39-40 ℃ respectively.