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Valve knowledge I Heat treatment of the ‘four fires’

一、Overview

In the valve industry, many parts and materials are involved in heat treatment, valve body, spool, seat welding and sealing surface surfacing, welding of valves and pipelines, etc., heat treatment is also a must for us to understand the knowledge, at least to have a general perception;

Heat treatment refers to the material in the solid state, by means of heating, insulation and cooling, in order to obtain the expected organisation and properties of a metal thermal processing technology, that is, to improve the intrinsic quality of the workpiece can not be seen by the naked eye.

Machining is surgery, heat treatment is internal medicine, representing the core competitiveness of a country's manufacturing industry.

Heat treatment, in contrast to other machining processes, generally does not change the shape and overall chemical composition of the workpiece, but rather imparts or improves the use of the workpiece by changing the internal microstructure of the workpiece, or by changing the chemical composition of the surface of the workpiece.

When it comes to heat treatment, most people think of the overall heat treatment (with surface heat treatment and chemical heat treatment side by side of the three categories), the application is also the most common, the overall heat treatment of roughly annealing, normalising, quenching and tempering of the four basic processes, that is, heat treatment of the ‘four fires’;

Regardless of which fire, are inseparable from the three processes of heating, holding, cooling. These three processes throughout all the heat treatment process, the three processes of good and bad decision of the final heat treatment quality.

二、The process

1, Heating

The purpose of heating is to obtain the desired high-temperature metallographic organisation, thereby changing the metallographic properties;

Heating temperature varies with the treated metal material and the purpose of heat treatment, but generally are heated to the phase transition temperature above;

Heating temperature is one of the important process parameters of the heat treatment process, choose a good heating temperature and control the heating time, is to ensure the quality of heat treatment of the main factors.

2, Insulation

The purpose of insulation is mainly two: one, is to make the workpiece completely heat through, heating uniformly, inside and outside the same, especially a very large workpiece, the innermost (core) temperature than the surface layer of the temperature of the heating rate is inconsistent, slower;

Secondly, to the metallurgical organisation transformation reserved enough time, so that the high temperature organisation transformation completely into the required metallurgical organisation;

3, Cooling

Cooling is an indispensable process in the heat treatment process, the cooling method is different due to different processes, different cooling rate also determines which it belongs to the fire.

三、The four fire distinction and memory

1, Quenching (cuìhuo), because the accent is similar to annealing, many old engineers spoken often called quenching (zhànhuo), the shortest cooling time, often water-cooled or oil-cooled; quenching the word, next to the three-pointed water, indicating that it is related to the liquid, and heat treatment dealing with fewer engineers and technicians are often very difficult to remember to distinguish between the four fire, but quenching should be the The best to remember (memory method: three-point water quenching, water-cooled, the fastest cooling rate);

2, Normalising, cooling rate between quenching and annealing, generally air-cooled; normalising can be slightly faster cooling in the crystalline grain refinement of the steel, not only to get satisfactory strength, but also can significantly improve toughness and reduce the tendency of the component to crack. (Memory method: normalising, positive gas, gas cooling, cooling rate in general);

3, Annealing, the slowest cooling rate, generally furnace cooling; the purpose is to reduce the hardness, improve the cutting and machinability; reduce the residual stress, stabilise the size, reduce the deformation and cracking tendency; refine the grain, adjust the organisation, eliminate organisational defects (memory method: annealing, slow annealing, slow cooling, the slowest cooling rate);

4, Tempering, is the quenched workpiece reheated to below the lower critical temperature (heating the pearlite to austenite transition to the beginning of the temperature) of the appropriate temperature, insulation for a period of time in the air or water, oil and other media cooling, and some will also have a second or more tempering (memory: tempering, go back to the treatment, the fastest cooling rate quenching is not dealt with, so the need for tempering);

‘Quenching’ and then ‘high-temperature tempering’ combination of two fire heat treatment is also known as the customary tempering treatment;