Quenching crack of failure steel caused by improper heat treatment

Defects and anomalies may occur in any heat treatment process associated with cold extrusion or forming, such as homogenization, preheating and annealing. Overheating and burning can reduce strength and toughness.
Another important reason of fracture may be caused by abnormal defects during surface hardening of parts. For example, in a case hardened part subjected to cyclic bending or torsion loading, fatigue cracks often begin at the interface between the surface and the center (where the strength gradient is large).
If preventive measures are not taken, the surface hardening treatment may affect the toughness obtained by true long color display. For example, sharp notch in parts requiring good toughness shall not be carburized or nitrided. If the parts need surface hardening, the notch area shall be covered or the hardened layer in the notch area shall be removed by machining i.e. grinding.
The toughness and toughness of the last kind of heat treatment or toughness required by heat treatment. Quenching cracks or embrittlement may occur in different heat treatment processes.

Quenching cracking of steel

The quenching cracks in steel originate from the stress produced by the increase of volume during the transformation of austenite to martensite. The martensite in the quenched state is very hard and has little ductility.
When parts made of hardenable alloy steel are quenched, martensite is first formed in the outermost layer which first reaches MS temperature. Due to the expansion of martensite, “work” is done to the softer austenite below, and the expansion of martensite on the outer surface is almost unlimited. When the material near the center reaches Ms Temperature and continues to cool down, the expansion of the new martensite is limited by the outer martensite which has already been formed, resulting in the internal stress that makes the surface open. When the internal stress generated by the mass formation of martensite is greater than the tensile strength of the martensite in the quenching state of the outer layer of the part, the cracking occurs.

Quenching cracks have several easily recognizable features

1. Generally, the fracture appears as a straight line, and the crack can be opened or extended by looking at the center from the surface, and the shear lip may appear on the edge surface.

2. Because the quenching crack occurs at a relatively low temperature, no decarburization is found in the macroscopic or microscopic examination.

3. The fracture surface is of fine structure. If tempered after quenching, the fracture surface may be blackened by oxidation. (area a in the figure is the quenching crack propagation section)

4. After tempering under high temperature and oxidation conditions, the quenching cracks were examined by microscopic examination, and tempered oxide scale was found.

Factors controlling cracking

1. Stress concentration:
Any state of stress concentration encountered in quenching will accelerate the formation of quenching cracks. Therefore, it is necessary to avoid or block the parts with sharp changes in cross-section such as rectangular keyways or holes during quenching. Through the new design to change the position of the sharp change of the punch section in the figure below, the method is to process small T-shaped cross-section and conduct heat treatment separately. It has been found that the cold stamping marks used to identify parts have also become the source of quenching cracks.
2. Cooling rate:
The distribution of part mass and lack of uniform or concentric cooling may also accelerate quenching cracking.
The faster the quenching agent is cooled, the more the hardening depth can be increased. Therefore, the selection of appropriate quenchant is often an important factor to eliminate quenching cracks. The most commonly used quenchants are caustic solution, molten salt, water, oil and air. The quenching speed is the fastest in caustic solution and the slowest in static air. Low melting point salts or metals are also used for more complex quenching treatments. For example, austempering of martensite and Austempering of austenite.
The heat treatment procedure, quenching agent and tempering procedure are selected according to the following factors:

• The hardness and mechanical properties of the parts for the best service life are given. The specific microstructure can be determined according to the performance requirements;
• The alloy steel selected for the parts determines the heat treatment properties of the required properties;
• Equipment of heat treatment workshop;
• The economy of heat treatment production should be considered according to the specific conditions of the workshop.

Since quenching and tempering (essentially different from normalizing or annealing products) are the most important and common heat treatment methods for steel, it is necessary to understand the basic factors to be considered when selecting specific heat treatment procedures and quenchants in order to obtain the required microstructure. In most cases, if the required properties are not achieved, the service life will be reduced The quenching agent should be selected according to the quenching speed, that is, the cooling rate that the quenching agent can achieve is enough to produce the required microstructure. It is important that after quenching, the workpiece should be tempered as soon as possible to remove the internal stress generated during quenching. It is better to take the workpiece out of the quenchant and put it into the tempering furnace when it is still in the thermal state of about 60 ~ 90 ℃. When the parts are taken out of the quenchant during oil quenching, the tempering furnace should smoke slowly.

Delayed cracking:

It is a common misconception that the workpiece can produce quenching crack only in quenchant. The cracks of some parts do not occur one hour after quenching, even one day after quenching.

Cause of quenching crack

Some common causes of quenching cracks in steel are as follows:

• It is overheated during austenitizing treatment to coarsen normal fine grain steel. Coarse grain steel is more sensitive to quenching cracking than fine grain steel;
• Improper selection of quenchants, for example, water, salt or alkali solution is used, which is originally the most suitable quenching agent for oil for specific parts and steel grades;
• Improper selection of steel grade;
• The time between quenching and tempering is too long;
• The allowable drop temperature of parts before tempering is too low (mainly applicable to hypereutectoid steel and tool steel);
• The design of keyway, hole, section mutation and stress concentration is not appropriate;
• The shape of parts is not considered during quenching, resulting in uneven or eccentric cooling;
• The items listed above will be more important to some steel grades than others in determining whether a given part will crack during quenching.

Even if the parts will not crack during quenching, if the residual stress acts in the same direction as the external load, the parts with local residual stress concentration after heat treatment to high strength (high hardness) may break instantaneously in service.

Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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