Screw heat treatment, also known as screw hardening.
Screws are divided into iron and stainless steel screws. Iron screws generally need to be harder and all of them need to be hardened. But stainless steel screws rarely need to be hardened because their hardness is already sufficient.
Now let’s take a look at the heat treatment methods for iron screws.
I. For heat treatment methods, different methods can be selected based on the purpose and objective.
Quenching and tempering steel: high-temperature tempering (500-650°C) after quenching. Spring steel: medium-temperature tempering (420-520°C) after quenching.
Carburized steel: low-temperature tempering (150-250°C) after carburizing and quenching. For low-carbon and carbon (alloy) steel quenched to martensite, as the tempering temperature increases, the general rule is that the strength decreases and the toughness increases.
However, due to the different carbon content in carbon steel, low and medium carbon tempering temperatures have different effects. Therefore, to obtain good comprehensive mechanical properties, the following methods can be adopted separately:
(1) Select low-carbon (alloy) steel and temper at a low temperature below 250°C after quenching to obtain low-carbon martensite. To improve the surface wear resistance of this type of steel, only the carbon content of each surface layer can be increased, that is, surface carburizing, generally known as carburized structural steel.
(2) Use medium-carbon steel with a higher carbon content, temper at a high temperature (500-650°C) after quenching (i.e., so-called quenching and tempering treatment), so that it can maintain sufficient strength under high plasticity. This type of steel is generally called quenched and tempered steel.
If high strength is desired and plasticity and toughness can be sacrificed, low-temperature tempering can be adopted for low-carbon alloy quenched and tempered steel, resulting in so-called “ultra-high strength steel”.
(3) For steel grades with carbon content between medium-carbon and high-carbon (such as 60, 70 steel) and some high-carbon steels (such as 80, 90 steel), if they are used to manufacture springs, to ensure high elastic limit, yield limit and fatigue limit, medium-temperature tempering after quenching is adopted.
II. Work process
(1) Quenched and tempered steel
1. Pre-treatment: normalizing → annealing (pearlitic type steel) → high-temperature tempering (martensitic type steel). The purpose of normalizing is to refine the grain, reduce the degree of banded structure in the organization, and adjust the hardness to facilitate mechanical processing. After normalizing, the steel has equiaxed fine grains.
2. Quenching: Heat the cylinder to about 850°C for quenching. The quenching medium can be selected based on the size of the steel part and the quenchability of the steel. Generally, water, oil or even air quenching can be chosen. Steel in the quenched state has low plasticity and high internal stress.
3. Tempering:
① To make the steel have high plasticity, toughness and appropriate strength, temper at about 400-500°C. For steels with high sensitivity to tempering brittleness, rapid cooling must be carried out after tempering to inhibit the occurrence of tempering brittleness.
② If the part requires particularly high strength, temper at about 200°C to obtain medium-carbon tempered martensite structure.
(2) Spring steel
1. Quenching: Oil quenching at 830-870°C.
2. Tempering: Temper at about 420-520°C to obtain tempered troostite structure.
(3) Carburized steel
1. Carburizing: A type of chemical heat treatment, which refers to the process of introducing C elements into the surface of steel parts in an active medium containing certain chemical elements at a certain temperature. It is divided into preheating (850°C), carburizing (890°C), and diffusion (840°C) processes.
2. Quenching: For carbon and low-alloy carburized steel, direct quenching or single quenching is generally adopted.
3. Tempering: Low-temperature tempering to eliminate internal stress and improve the strength and toughness of the carburized layer.
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