Heat Treatment-1

Definition

The heat treatment process of steel refers to the specific heating, insulation and cooling process parameters of steel during heat treatment, which are formulated according to the structural transformation law of steel during heating and cooling.

Classification

According to the heating and cooling methods and the different structures and properties obtained, the heat treatment process of steel can be divided into:

(1) Ordinary heat treatment (annealing, normalizing, quenching and tempering)

(2) Surface heat treatment (surface quenching and chemical heat treatment, etc.)

(3) Special heat treatment (deformation heat treatment, magnetic field heat treatment, etc.).

According to the position and role of heat treatment in the process flow of parts production, heat treatment can be divided into:

(1) Preliminary heat treatment, mainly annealing, normalizing, etc.

(2) Final heat treatment, mainly quenching, tempering, etc.

Annealing and normalizing of steel

Annealing

The heat treatment process of heating steel with a structure deviating from the equilibrium state to an appropriate temperature,

insulating it for a certain period of time, and then slowly cooling it to obtain a structure close to the equilibrium state is called annealing.

Purpose

Eliminate coarse columnar crystals and equiaxed crystals, refine grains, reduce hardness, improve cutting performance, and make the structure uniform.

Classification

There are many types of steel annealing processes, which can be divided into two categories according to the heating temperature:

1. Phase transformation recrystallization annealing, including complete annealing, incomplete annealing,

spheroidizing annealing and diffusion annealing.

2. Annealing below the critical temperature, including recrystallization annealing and stress relief annealing.

Normalizing can be regarded as a special form of annealing, and the cooling method is air cooling.

1 Full annealing

Full annealing is a heat treatment process that heats steel to 30-50℃ above Ac3 temperature, keeps the temperature for a sufficient time, makes the structure completely austenitized, and then slowly cools it with the furnace to obtain a heat treatment process close to the equilibrium structure.

The purpose is to refine the grains, make the structure uniform, eliminate internal stress and hot processing defects, reduce hardness, and improve cutting performance and cold plastic deformation performance.

The structure undergoes recrystallization, making the grains refined, the structure uniform, and the Widmanstätten structure and banded structure eliminated. For forgings and rolling parts, full annealing is arranged after the workpiece is hot forged and hot rolled, and before cutting: for welded parts or castings, it is generally arranged after welding and pouring (or after diffusion annealing).

Heating temperature and time The temperature is generally 20-30℃ above Ac3. The annealing holding time depends not only on the time required for the workpiece to burn through (that is, the core of the workpiece reaches the required temperature), but also on the time required for the structure transformation. The full annealing holding time is related to the chemical composition of the steel, the shape and size of the workpiece, the type of heating equipment, the amount of furnace loading and the method of loading. Usually the heating time is calculated based on the effective thickness of the workpiece.

Cooling method Furnace cooling, carbon steel <200 ℃/h, low alloy steel <100 ℃/h, high alloy steel <50 ℃/h. Furnace temperature <600 ℃.
Organization: Lamellar pearlite

Annealing process of carbon steel castings and hardness after annealing

Carbon content	Annealing temperature	heat preservation		Cooling method	Hardness(HBS)
		Casting thickness/mm	time/h
0.10-0.20	910-880	＜30	1	the furnace cools to 620℃——air-cooled	115-143
0.20-0.30	880-850	＜30	1	the furnace cools to 620℃——air-cooled	1333-156
0.30-0.40	850-820	＞30	Every 30mm increase, add 1h	the furnace cools to 620℃——air-cooled	143-178
0.40-0.50	820-800	＞30	Every 30mm increase, add 1h	the furnace cools to 620℃——air-cooled	156-217
0.50-0.60	800-780	＞30	Every 30mm increase, add 1h	the furnace cools to 620℃——air-cooled	187-230