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Hot rolling is carried out above the recrystallization temperature, while cold rolling is carried out below the recrystallization temperature. Sometimes, cold rolling may also involve heating, but the temperature is relatively low. After cold rolling, work hardening occurs, so if the forming requirements of the material are relatively high, annealing is also required.
Cold rolling and hot rolling are generally used for sheet metal or section steel, while cold drawing is generally used for wire with a circular cross-section. In addition, hot-rolled plates are generally high-alloy, high-strength steel, while cold-rolled plates are low-carbon, low-alloy steel. Cold rolling can improve strength and ensure surface quality of materials.
Cold-drawn tubes are divided into circular tubes and special-shaped tubes:
Cold-drawn steel pipes usually require multiple drawing operations, and there must be corresponding stress relief annealing between each drawing operation to ensure the smooth progress of the next cold drawing operation.
From the appearance, cold-drawn seamless steel pipes are often small-diameter, while hot-rolled seamless steel pipes are often large-diameter. The precision of cold-drawn seamless steel pipes is higher than that of hot-rolled seamless steel pipes, and the price is also higher.
Cold-drawn seamless pipes generally have a smaller diameter, mostly below 127mm. Especially, the outer diameter precision of cold-drawn seamless pipes is very high, and the length of cold-drawn seamless pipes is generally shorter than that of hot-rolled seamless pipes. In terms of wall thickness, the wall thickness of cold-drawn seamless pipes is more uniform than that of hot-rolled seamless pipes.
It is a heat treatment process that heats the quenched workpiece to an appropriate temperature below the lower critical temperature, holds it for a period of time, and then cools it to room temperature in air or in a medium such as water or oil.
The purpose of tempering :
To eliminate residual stress generated during quenching of the workpiece, and prevent deformation and cracking ;
To adjust the hardness, strength, plasticity, and toughness of the workpiece to meet the performance requirements ;
To stabilize the structure and size and ensure accuracy ;
To improve and enhance the processing performance.
Meaning : Refers to tempering of the workpiece between 150~250℃.
Purpose : To maintain the high hardness and wear resistance of the quenched workpiece, reduce quenching residual stress and brittleness. After tempering, tempered Martensite is obtained, which refers to the structure obtained when Martensite is tempered at low temperature.
Application : Cutting tools, measuring tools, molds, rolling bearings, carburizing and surface hardened parts, etc.
Meaning : Refers to tempering of the workpiece between 350~500℃.
Purpose : To obtain higher elasticity and yield point, and suitable toughness. After tempering, tempered Austenite is obtained, which refers to the dual-phase structure in which extremely fine spherical carbides (or carburized bodies) are distributed in the ferrite matrix formed when Martensite is tempered.
Application : Springs, forging dies, impact tools, etc.
Meaning : Refers to tempering of the workpiece above 500℃.
Purpose : To obtain good comprehensive mechanical properties of strength, plasticity, and toughness. After tempering, tempered Sorbite is obtained, which refers to the dual-phase structure in which fine spherical carbides (including carburized bodies) are distributed in the ferrite matrix formed when Martensite is tempered.
Application : Widely used in various important force components, such as connecting rods, bolts, gears, and shaft parts, etc.
