6 heat treatment issues to help you understand heat treatment!

Time：2020-03-24 09:14:48 / Popularity： / Source：

First, what is annealing of steel? What are types of annealing and what are its uses?

Annealing of steel:

Annealing is a heat treatment process in which steel is heated to a temperature above or below critical point AC1 and then slowly cooled with furnace after a certain period of time to obtain a near-equilibrium structure.

Annealing type:

According to heating temperature, it can be divided into annealing above or below critical temperature AC1. The former includes complete annealing, incomplete annealing, spheroidizing annealing and homogenization annealing, the latter includes recrystallization annealing and stress relief annealing. According to cooling method, it can be divided into isothermal annealing and continuous cooling annealing.

Annealing use:

1. Complete annealing

Complete annealing is a heat treatment process in which steel is heated to a temperature of 20-30℃ above AC3 for a long time to make microstructure completely austenitized and then slowly cooled with furnace to obtain a near-balanced state. It is mainly used in hypoeutectoid steel, which aims to refine grains, eliminate internal stress and work hardening, improve plastic toughness, uniform steel chemical composition and structure, improve cutting performance of steel, eliminate defects such as Wei's structure and band structure in medium carbon structural steel.

2. Incomplete annealing

Incomplete annealing is a heat treatment process in which steel is heated to between AC1-AC3 (hyp-eutectoid steel) or AC1-ACcm (hyper-eutectoid steel), and after a certain period of time, furnace is slowly cooled to obtain a near-equilibrium structure. For hypoeutectoid steel, if original microstructure of steel is properly distributed, incomplete annealing may be used instead of full annealing to eliminate internal stress and reduce hardness. For hypereutectoid steel, incomplete annealing is mainly to obtain spherical pearlite structure to eliminate internal stress, reduce hardness and improve cutting performance.

3. Spheroidizing annealing

Spheroidizing annealing is a heat treatment process for spheroidizing carbides in steel to obtain granular pearlite. Mainly used in eutectoid steel, hypereutectoid steel and alloy tool steel. Its purpose is to reduce hardness, improve cutting performance, uniform organization, and preparation for quenching.

4. Homogenization Annealing

Also known as diffusion annealing, it is a heat treatment process in which steel ingots, castings or forged billets are heated to a temperature slightly lower than solidus temperature for a long time and then slowly cooled to room temperature. Its purpose is to eliminate dendrite segregation and regional segregation during solidification process of ingot or casting, homogenize composition and structure.

5. Recrystallization annealing

A heat treatment process in which cold deformed metal is heated to a temperature above recrystallization temperature for a suitable period of time and then slowly cooled to room temperature. Its purpose is to re-transform deformed grains into uniform equiaxed grains, while eliminating work hardening and residual internal stress, returning microstructure and properties of steel to state before cold deformation.

6. Stress relief annealing

A heat treatment process in which a cold deformed metal is heated to a temperature below recrystallization temperature, held for a while and then slowly cooled to room temperature. Its main purpose is to eliminate residual internal stress (mainly the first type of internal stress) in castings, forged parts, welded parts and machined parts to improve dimensional stability, reduce tendency of workpiece deformation and cracking.

Second, what is normalization of steel? What is purpose? What is application?

Normalizing of steel:

Normalizing is a heat treatment process in which steel is heated to a suitable temperature above AC3 or Accm, after austenitization at a suitable time for cooling at a relatively rapid rate (air cooling, air cooling or spraying) to obtain a pearlite-like structure. Essence of normalizing process is complete austenitization plus pseudoeutectoid transformation.

Objective:

To refine grain, uniform composition and structure, eliminate internal stress, adjust hardness, eliminate defects such as Wei's structure, band structure and network carbide, provide a suitable tissue state for final heat treatment.

Application:

1. Improve cutting performance of low carbon steel.
2. Eliminate thermal processing defects (Wei's structure, banded structure, coarse grains) of medium carbon steel.
3. Eliminate network carbide of hypereutectoid steel, facilitate spheroidizing annealing, and prepare structure for quenching.
4. As a final heat treatment, improve mechanical properties of ordinary structural parts.

Third, in order to improve strength of hypo-eutectoid steel in production, common method is to increase content of pearlite in hypo-eutectoid steel. What heat treatment process should be used?

A normalizing process should be used.

Reason:

Sub-eutectoid ferrite is precipitated in super-cooled austenite during cooling process. The slower cooling rate, the more content of pro-eutectoid ferrite is, which resulting in less pearlite content, lowering hardness and strength of hypoeutectoid steel. Essence of normalizing process is complete austenitization plus pseudoeutectoid transformation, which can reduce content of pro-eutectoid ferrite by increasing cooling rate, and transform steel of hypoeutectoid composition into eutectoid structure, that is, increase pearlite content, thereby increasing strength and hardness of hypoeutectoid steel.

Forth, what is purpose of quenching? How many kinds of quenching methods are there? Compare advantages and disadvantages of several quenching methods?

Purpose of quenching:

To obtain as much martensite as possible, can significantly improve strength, hardness and wear resistance of steel. Combined with various tempering processes, steel can have good strength and toughness while having high strength and high hardness.
Heat treatment process of heating steel to a critical point AC3 or a certain temperature above AC1, then cooling at a cooling rate greater than critical cooling rate to obtain martensite (or lower bainite) is called quenching.

Quenching method:

According to cooling method, it can be divided into: single liquid quenching method, two-liquid quenching method, fractional quenching method, isothermal quenching method

Advantages and disadvantages:

Quenching method	Advantages	Disadvantages
Single-liquid quenching method	Simple operation and wide application	1. Only suitable for small-sized and simple-shaped workpieces 2. Large quenching stress 3. It is not easy to select a cooling medium with suitable cooling capacity and cooling characteristics.
Two-liquid quenching method	1. Reduce stress of structure, reduce tendency of workpiece deformation and cracking. 2, Suitable for large workpieces	Operation is not easy to control, requires extensive experience and skilled technology
Graded quenching method	1. Reduce thermal stress and microstructure stress, reduce tendency of workpiece deformation and cracking 2, operation is relatively easy to control	Only for smaller workpieces
Austempering method	1. Reduce thermal stress and microstructure stress, significantly reduce workpiece deformation and cracking tendency 2, suitable for processing workpieces with complex shapes and precise dimensions	Only for smaller workpieces

Fifth, selection principle of quenching heating temperature of yawed steel and hypereutectoid steel. Why does hypereutectoid steel quenching heating temperature not exceed Accm line?

Principle of quenching heating temperature selection

Based on principle of obtaining uniform fine austenite grains to obtain a fine martensite structure, Hypoeutectoid steel is usually heated to 30-50℃ above AC3, and hypereutectoid steel is heated to above AC1 30-50℃.
1. Quenching heating temperature of hypereutectoid steel exceeds Accm line, carbides are all dissolved in austenite, so that increases carbon content of austenite, lowers Ms and Mf points of steel, increases the amount of retained austenite after quenching, which reduces hardness and wear resistance of steel.
2. Quenching temperature of hypereutectoid steel is too high, austenite grains are coarsened and carbon content is high. After quenching, coarse needle-shaped martensite with microcracks is easily obtained, which reduces plastic toughness of steel.
3. During quenching at high temperature, quenching thermal stress is large, and oxidative decarburization is serious, which also increases tendency of steel deformation and cracking.

Six, what is hardenability and hardening capacity of steel? What are factors affecting hardenability, hardening capacity and depth of hardened layer of steel?

Hardenability:

Hardenability of steel refers to ability of austenitized steel to obtain martensite during quenching. It reflects stability of supercooled austenite and is related to critical cooling rate of steel. Its size is expressed by depth and hardness distribution of hardened layer obtained by quenching steel under certain conditions.

Hardening capacity:

Hardening capacity of steel refers to ability of steel after austenitization to harden during quenching, mainly depending on carbon content in martensite, expressed by the highest hardness that can be achieved by quenched martensite.

Depth of hardened layer:

Depth of hardened layer refers to depth of semi-martensitic zone to surface of workpiece measured when steel is quenched under specific conditions. It is related to hardenability of steel, shape and size of workpiece, cooling capacity of quenching medium.

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