THE DIFFERENCE BETWEEN BK, GBK, BKS, NBK IN STEEL.

THE DIFFERENCE BETWEEN BK, GBK, BKS, NBK IN STEEL.

ABSTRACT:

Annealing and normalizing of steel are two common heat treatment processes.
Preliminary heat treatment purpose: to eliminate some defects in the blanks and semi-finished products, and prepare the organization for the subsequent cold working and final heat treatment.
The final heat treatment purpose: to obtain the required performance of the workpiece.
The purpose of annealing and normalizing is to eliminate certain defects caused by hot processing of steel, or to prepare for the subsequent cutting and final heat treatment.

 

 Annealing of steel:
1. Concept: The heat treatment process of heating steel parts to an appropriate temperature (above or below Ac1), keeping it for a certain period of time, and then slowly cooling to obtain a structure close to equilibrium is called annealing.
2. Purpose:
(1) Reduce hardness and improve plasticity
(2) Refine grains and eliminate structural defects
(3) Eliminate internal stress
(4) Prepare the organization for quenching
Type: (According to the heating temperature, it can be divided into annealing above or below the critical temperature (Ac1 or Ac3). The former is also called phase change recrystallization annealing, including complete annealing, diffusion annealing homogenization annealing, incomplete annealing, and spheroidizing annealing; The latter includes recrystallization annealing and stress relief annealing.)

  •  Complete annealing(GBK+A) :

1) Concept: Heat the hypoeutectoid steel (Wc=0.3%~0.6%) to AC3+(30~50)℃, and after it is completely austenitized, heat preservation and slow cooling (following the furnace, burying in sand, lime), The heat treatment process to obtain a structure close to the equilibrium state is called complete annealing. 2) Purpose: Refine grains, uniform structure, eliminate internal stress, reduce hardness, and improve cutting performance.
2) Process: complete annealing and slow cooling with the furnace can ensure the precipitation of proeutectoid ferrite and the transformation of supercooled austenite into pearlite in the main temperature range below Ar1. The holding time of the workpiece at the annealing temperature not only makes the workpiece burn through, that is, the core of the workpiece reaches the required heating temperature, but also ensures that all homogenized austenite is seen to achieve complete recrystallization. The holding time of complete annealing is related to factors such as steel composition, workpiece thickness, furnace loading capacity and furnace loading method. In actual production, in order to improve productivity, annealing and cooling to about 600 ℃ can be out of the furnace and air cooling.
Scope of application: casting, welding, forging and rolling of medium carbon steel and medium carbon alloy steel, etc. Note: Low carbon steel and hypereutectoid steel should not be fully annealed. The hardness of low carbon steel is low after being fully annealed, which is not conducive to cutting processing. When the hypereutectoid steel is heated to austenite state above Accm and slowly cooled and annealed, a network of secondary cementite is precipitated, which significantly reduces the strength, plasticity and impact toughness of the steel.

  • Spheroidizing annealing:

1) Concept: The annealing process to spheroidize carbides in steel is called spheroidizing annealing.
2) Process: General spheroidizing annealing process Ac1+(10~20)℃ is cooled with furnace to 500~600℃ with air cooling.
3) Purpose: reduce hardness, improve organization, improve plasticity and cutting performance.
4) Scope of application: mainly used for cutting tools, measuring tools, molds, etc. of eutectoid steel and hypereutectoid steel. When the hypereutectoid steel has a network of secondary cementite, it not only has high hardness and is difficult to perform cutting, but also increases the brittleness of the steel, which is prone to quenching deformation and cracking. For this reason, a spheroidizing annealing process must be added after the hot working of the steel to spheroidize the flake infiltrate in the reticulated secondary cementite and pearlite to obtain granular pearlite.
Cooling rate and isothermal temperature will also affect the effect of carbide spheroidization. Fast cooling rate or low isothermal temperature will cause pearlite to be formed at a lower temperature. The carbide particles are too fine and the aggregation effect is small, which makes it easy to form flaky carbides. As a result, the hardness is high. If the cooling rate is too slow or the isothermal temperature is too high, the carbide particles formed will be coarser and the agglomeration effect will be very strong. It is easy to form granular carbides of varying thickness and make the hardness low.

  •  Homogenization annealing (diffusion annealing):

1) Process: The heat treatment process of heating alloy steel ingots or castings to 150~00℃ above Ac3, holding for 10~15h and then slowly cooling to eliminate the uneven chemical composition.
2) Purpose: Eliminate dendrite segregation during crystallization and homogenize the composition. Due to the high heating temperature and long time, the austenite grains will be severely coarsened. Therefore, it is generally necessary to perform a complete annealing or normalizing to refine the grains and eliminate overheating defects.  
3) Scope of application: mainly used for alloy steel ingots, castings and forgings with high quality requirements.  
4) Note: High temperature diffusion annealing has a long production cycle, high energy consumption, serious oxidation and decarburization of the workpiece, and high cost. Only some high-quality alloy steels and alloy steel castings and steel ingots with severe segregation use this process. For castings with small general sizes or carbon steel castings, because of their lighter degree of segregation, complete annealing can be used to refine grains and eliminate casting stress.

  • Stress relief annealing 

1) Concept: Annealing to remove the stress caused by plastic deformation processing, welding, etc. and the residual stress in the casting is called stress relief annealing. (No distortion occurs during stress relief annealing) 
2) Process: slowly heat the workpiece to 100~200℃ (500~600℃) below Ac1 and keep it for a certain period of time (1~3h), then slowly cool it to 200℃ with the furnace, and then cool it out of the furnace.  
Steel is generally 500~600℃ 
Cast iron generally exceeds 550 buckles at 500-550 ℃, which will easily cause the graphitization of pearlite.  Welding parts are generally 500~600℃.
3) Scope of application: Eliminate residual stress in cast, forged, welded parts, cold stamped parts and machined workpieces to stabilize the size of steel parts, reduce deformation and prevent cracking.

Normalizing of steel:
1. Concept: heating the steel to 30-50°C above Ac3 (or Accm) and holding it for a proper time; the heat treatment process of cooling in still air is called normalizing of steel.  
2.  Purpose: Refine grain, uniform structure, adjust hardness, etc.  
3. Organization: Eutectoid steel S, hypoeutectoid steel F+S, hypereutectoid steel Fe3CⅡ+S 
4.   Process: Normalizing heat preservation time is the same as complete annealing. It should be based on the workpiece through burning, that is, the core reaches the required heating temperature, and factors such as steel, original structure, furnace capacity and heating equipment should also be considered. The most commonly used normalizing cooling method is to take the steel out of the heating furnace and cool it naturally in the air. For large parts, blowing, spraying and adjusting the stacking distance of steel parts can also be used to control the cooling rate of steel parts to achieve the required organization and performance.

5.  Application range:  

  • 1) Improve the cutting performance of steel. Carbon steel and low-alloy steel with a carbon content of less than 0.25% have lower hardness after annealing, and are easy to “stick” during cutting. Through normalizing treatment, free ferrite can be reduced and flake pearlite can be obtained. Increasing the hardness can improve the machinability of steel, increase the life of the tool and the surface finish of the workpiece. 
  • 2) Eliminate thermal processing defects. Medium-carbon structural steel castings, forgings, rolling parts and welded parts are prone to overheating defects and banded structures such as coarse grains after heating. Through normalizing treatment, these defective structures can be eliminated, and the purpose of grain refinement, uniform structure and elimination of internal stress can be achieved. 
  • 3) Eliminate the network carbides of hypereutectoid steel, facilitating spheroidizing annealing. Hypereutectoid steel should be spheroidized and annealed before quenching to facilitate machining and prepare the structure for quenching. However, when there are serious network carbides in the hypereutectoid steel, a good spheroidizing effect will not be achieved. Net carbide can be eliminated by normalizing treatment. 
  • 4) Improve the mechanical properties of common structural parts. Some carbon steel and alloy steel parts with little stress and low performance requirements are normalized to achieve a certain comprehensive mechanical performance, which can replace quenching and tempering treatment as the final heat treatment of the parts.

Choice of annealing and normalizing  
The main difference between annealing and normalizing:  
1. The cooling rate of normalizing is slightly faster than annealing, and the degree of undercooling is greater. 
2. The structure obtained after normalizing is finer, and the strength and hardness are higher than that of annealing. The choice of annealing and normalizing:

  • For low carbon steel with carbon content <0.25%, normalizing is usually used instead of annealing. Because the faster cooling rate can prevent the low carbon steel from precipitating free tertiary cementite along the grain boundary, thereby improving the cold deformation performance of the stamping parts; normalizing can improve the hardness of the steel and the cutting performance of the low carbon steel; In the heat treatment process, normalizing can be used to refine the grains and improve the strength of low carbon steel.
  • Medium carbon steel with carbon content between 0.25 and 0.5% can also be normalized instead of annealing. Although the hardness of medium carbon steel close to the upper limit of carbon content is higher after normalizing, it can still be cut and the cost of normalizing Low and high productivity. 
  • Steel with carbon content between 0.5 and 0.75%, due to the high carbon content, the hardness after normalizing is significantly higher than that of annealing, and it is difficult to cut. Therefore, full annealing is generally used to reduce the hardness and improve cutting. Processability. 
  • High carbon steels or tool steels with carbon content> 0.75% generally use spheroidizing annealing as a preliminary heat treatment. If there is a network of secondary cementite, it should be normalized first.

Source:Mechanical professional literature.

Editor:Ali

 


Post time: Oct-27-2021