High manganese steel castings after water hardness treatment, the initial hardness is low, what is the cause of magnetic?

High manganese steel castings often have an initial hardness lower than Brinell 180 after water toughness treatment, and there may also be magnetization phenomenon when adsorbed by magnets. So what is the reason for this result? What impact does this have on the quality of castings? How can we solve this problem in production.

What is the reason for the low initial hardness and magnetism of a high manganese steel casting after water toughness treatment? How to improve? High manganese steel castings have low hardness and magnetism after water toughening treatment, mainly due to improper heat treatment processes or compositional deviations. The specific reasons are as follows:

Heat treatment process issues

1. Insufficient heating temperature or short holding time

The water toughening treatment of high manganese steel (such as ZGMn13) requires heating to 1050-1100 ℃ to fully dissolve the carbides into the austenite. If the temperature is not sufficient or the holding time is not sufficient, the carbides are not completely dissolved, which will lead to a low carbon content in the austenite matrix, a decrease in hardness (the normal hardness after water toughening should be ≥ HB200), and undissolved carbides may induce the formation of a small amount of ferrite, producing magnetism.

2. Insufficient cooling speed

After heating, rapid water cooling is required (water temperature ≤ 30 ℃). If the cooling rate is slow (such as insufficient water volume or large casting thickness), austenite may precipitate carbides or transform into martensite or ferrite, resulting in a decrease in hardness and magnetic properties.

Chemical composition deviation

1. Low carbon content

The carbon content of high manganese steel is usually between 0.9% and 1.4%, and carbon is a key element in maintaining the stability of austenite. If the carbon content is low (such as<0.9%), the stability of austenite decreases, and ferrite is easily precipitated after water toughening treatment, resulting in insufficient hardness and magnetism.

2. Insufficient manganese content or influence from other elements

The manganese content should be ≥ 11% (such as ZGMn13 containing 11%~14% manganese). If the manganese content is too low, the stability of austenite decreases and ferrite is easily generated; In addition, excessive silicon content (>0.8%) may promote carbide precipitation and also affect tissue stability.

tissue defect

1. Excessive residual carbides

If the cooling rate of the casting is slow and the primary carbides are coarse and not completely dissolved in the water toughening treatment, the residual carbides will reduce the hardness of the matrix, and the austenite around the carbides may transform into ferrite due to uneven composition, resulting in magnetism.

2. Coarse austenite grains

Heating at too high a temperature or holding for too long can lead to coarsening of austenite grains, easy precipitation of carbides or formation of ferrite at grain boundaries, affecting hardness and magnetism.

Other factors

Uneven wall thickness of castings: slow cooling rate in thick areas, which can easily form non austenitic structures;

Water quality issue: Poor water quality (such as impurities and high water temperature) during water cooling reduces cooling efficiency and leads to insufficient tissue transformation.

Solution measures

1. Optimize heat treatment process: Ensure heating temperature (1050-1100 ℃) and insulation time (usually 1-2 hours/25mm based on wall thickness calculation), and use sufficient low-temperature water for rapid cooling;

2. Control chemical composition: Adjust the carbon (0.9%~1.4%) and manganese (11%~14%) content according to standards, with silicon ≤ 0.8%;

3. Re water toughening treatment: Conduct secondary water toughening treatment on unqualified castings to remove residual carbides;

4. Casting process improvement: Control the pouring temperature and cooling rate to reduce the formation of primary carbides.

If the problem persists, it is recommended to test the chemical composition and metallographic structure, and adjust the process accordingly.

What are the effects of magnetism on the quality of high manganese steel castings with low initial hardness after water toughness treatment? High manganese steel castings have low hardness (<HB180) and magnetism after water toughening treatment, indicating the presence of non austenitic phases (such as ferrite, martensite, or residual carbides) in the structure. This can cause the following hazards to the performance and use of the castings:

Significant decrease in mechanical properties

1. Significantly reduced wear resistance

The wear resistance of high manganese steel depends on the characteristic of austenite structure transforming into martensite under impact load. If there is a large amount of ferrite or residual carbides in the organization, and the austenite content is insufficient, the martensitic transformation cannot be effectively induced under impact, and the wear rate will significantly increase (for example, when used for crusher liners, the service life may be shortened by more than 50%).

2. Insufficient strength and toughness

The presence of ferrite and carbides can fracture the austenite matrix, resulting in a decrease in tensile strength (normal ≥ 685MPa) and impact toughness (≥ 14J/cm ²), and castings are prone to plastic deformation or fracture under load (such as excavator bucket teeth cracking easily).

Deterioration of corrosion resistance and oxidation resistance

The electrode potential of ferrite is lower than that of austenite, and it is prone to form micro cells in corrosive media, accelerating electrochemical corrosion (such as pitting or rusting on the surface when used in acidic slurries);

The interface between residual carbides and the matrix is prone to become the starting point for oxidation, and the antioxidant capacity decreases at high temperatures (such as>300 ℃), leading to the formation of a loose oxide layer on the surface.

Possible safety hazards during use

1. Assembly problems caused by magnetism

Magnetic castings may adsorb impurities such as iron filings, which can affect the accuracy of operation or cause jamming in precision mechanical assembly (such as the drum of mineral processing equipment), and even lead to equipment failure.

2. Failure risk under dynamic loads

If components used to withstand impact, such as railway turnouts, have uneven organization, it can lead to stress concentration, which may cause crack propagation after short-term use and increase the risk of sudden fracture.

4. Increased costs for subsequent processing and maintenance

Castings with insufficient hardness cannot be directly used and require re water toughening treatment, which increases energy consumption and labor costs for heat treatment;

If the organizational defects are severe (such as a large amount of coarse carbides), secondary treatment may not be able to completely repair them and can only be scrapped, resulting in material waste.

summarize

The core performance of high manganese steel lies in its "single austenite structure". Low hardness and magnetism are direct manifestations of poor microstructure, which will weaken the value of castings in terms of wear resistance, mechanical properties, safety, and other aspects. Strictly control the heat treatment process and chemical composition during production to avoid such problems.