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What does the tempering brittleness of steel mean? What’s the solution?

  Tempered brittleness refers to the embrittlement of quenched steel in some tempering range or slowly cooling from tempering temperature through the temperature range. The tempering brittleness can be divided into the first kind of tempering brittleness and the second kind of tempering brittleness.

  The first type of tempering brittleness, also known as irreversible tempering brittleness, mainly occurs when the tempering temperature is 250 ~ 400, after the disappearance of reheating brittleness, repeated tempering in this interval, no longer occurs brittleness.

  The second type of tempering brittleness, also known as reversible tempering brittleness, occurs at the temperature of 400 ~ 650, when the reheating brittleness disappears, should be rapidly cooled, not in 400 ~ 650 interval for a long time to stay or slow cooling, otherwise, the catalytic phenomenon will occur again. The occurrence of tempering brittleness is related to the alloying elements contained in steel, such as manganese, chromium, silicon, and nickel, which tend to produce tempering brittleness, while molybdenum and tungsten tend to weaken the tempering brittleness.

  The tempering brittleness of spring first-class tempering brittle alloy steel after quenching in the range of 250 ~ 400 temperings, is intergranular fracture characteristics, and can not be eliminated by reheating method, so it is also called irreversible tempering brittleness.It is mainly produced in alloy structural steel. The first type of tempering brittleness is also called low tempering brittleness when tempering between 200 and 350. Such as in the first type of tempering brittleness and then heated to a higher temperature tempering, the brittleness can be eliminated, so that the impact toughness increases again.At this point, if the temperature range of 200 ~ 350 tempering will not produce such brittleness. Thus, the first kind of tempering brittleness is irreversible, so it can also be called irreversible tempering brittleness. The first type of tempering brittleness exists in almost all steels. For example, Cr-Mn steel with different carbon content has low impact toughness at 350 after tempering. The first kind of temper brittleness is not only to reduce the impact toughness at room temperature, but also make the ductile-brittle transition temperature 50% FATTe [the impact toughness of steel material with testing temperature decreased significantly decline when the corresponding temperature, even if the steel material from resilience state transition temperature is called the ductile-brittle transition temperature for the brittle state, with 50% FATT (), as shown in the metal mechanical properties] rise, fracture toughness KIe decline. Such as Fe-0.28C-0.64Mn-4.82Mo steel after 225 tempering KIe is 117.4Mn/m, and after 300 tempering due to the emergence of the first kind of tempering brittleness, so that KIe reduced to 73.5Mn/m.Most of the first type of tempering brittleness is an intergranular fracture, but a few are transgranular cleavage fractures.

  The high temperature tempering temperature is 500~600°C, and the temperature is kept for an appropriate time before cooling.It is mainly used for adjusting the structure of cast steel after quenching or normalizing to make carbon steel, low and medium alloy steel castings with high strength and good toughness. The tempering brittleness is a problem that must be paid attention to when making the tempering process of alloy steel castings. Occurs in the following two temperature ranges. Brittleness at 250~400°C: The quenched martensitic structure of cast steel, in this temperature range, will produce tempering brittleness.If the tempering temperature is slightly higher than the brittle zone, the tempering brittleness can be eliminated. And after tempering in the above temperature range, there will be no tempering brittleness, so it is often called the first type of tempering brittleness. Brittleness at 400~500°C (or even 650°C): This will happen to most low-alloy cast steels, that is, high temperature tempering brittleness of cast steel. Brittleness can be eliminated by heating steel castings that have produced brittleness within this temperature range to more than 600°C (or 650°C), followed by rapid cooling in water or oil. However, in castings where brittleness has been eliminated, brittleness will appear again if heated to the temperature at which the tempering brittleness occurs. This is often referred to as type II tempering brittleness.

  1. The first kind of tempering brittleness (also known as low temperature tempering brittleness or irreversible tempering brittleness) temperature range: 200~350oC

Causes:

  1.Harmful impurity elements S, P, As, Sn, Sb, Cu, H, O lead to the first type of tempering brittleness

  2. Mn, Si, Cr, Ni, V promote the first type of tempering brittleness, Nickel-Si coexistence also plays a role in promoting the tempering brittleness temperature chromium silicon

  3.The larger the austenite grain is, the more residual austenite is, and the more serious the tempering brittleness of the first type is

  4.The formation of impurity elements and thin carbide shell in austenitic grain boundary decreases the grain boundary strength

  Countermeasures:

  1.Do not temper in this temperature range

  2.Isothermal quenching is used instead

  3.Reduce impurity elements in steel

  4.Refine austenite grain

  2. The second kind of tempering brittleness (high temperature tempering brittleness, reversible tempering brittleness) temperature range: 450~650oC

Causes:

  1.Impurity elements P, Sn, Sb, As, B, S cause brittleness

  In nickel-chrome steel, antimony has the greatest influence, tin is the second

  In chrome-manganese steel, phosphorus plays the most important role, followed by antimony and tin

Phosphorus is more effective than tin for mild steel

The effect of tin on medium carbon steel is greater than that of phosphorus

  2.Promote the second type of tempering brittleness elements are Ni, Cr, Mn, Si, C, these elements and impurity elements at the same time cause brittleness

  When there is one element in steel, the brittleness of manganese is the highest, followed by chromium and nickel

  The presence of two elements at the same time makes the embrittlement greater

  3. Mo, W, V, Ti, and rare earth elements can resist tempering brittleness

  4. Too slow cooling rate after tempering causes brittleness

  5. The austenite grain is large

  6. The mechanism of brittleness is grain boundary precipitation and grain boundary segregation theory

Countermeasures:

  1. Reduce impurity elements in steel

  2. Nb, vanadium, and titanium are added to refine austenite grains

  3. Adding the second type of tempering brittleness of the elements molybdenum and tungsten

  4. Avoid tempering at 450~650oC, which should be quickly cooled after tempering

  5. Use of Sub-temperature Quenching and Casting Waste Heat Quenching to Reduce and Curl the Second Type of Tempering Brittleness (END)

  The tempering brittleness refers to the tempering process of steel after quenching. With the increase of tempering temperature, the hardness and strength of the steel matrix are reduced, while the plasticity and toughness are improved and improved. However, when tempering in a certain temperature range, the toughness with the rise of tempering temperature and there is a trough or decrease in the phenomenon, this phenomenon is called tempering brittleness. Generally, brittleness is caused by low tempering temperature or insufficient tempering time. It can be prevented and remedied by choosing reasonable tempering temperature and sufficient tempering. The brittleness of structural steel is shown in Fig. 1.In ordinary nickel and chromium steels, the tempering brittleness is very obvious. Steel in the process of tempering may be two types of brittleness: a brittle usually within the range of tempering temperature range 200 ~ 400, the longer the more obvious, and has nothing to do with the cooling speed after tempering, often appear in carbon steel and alloy steel, the temper brittleness even tempered, quick cold or reheated tempering are inevitable, known as the first kind of temper brittleness, also known as irreversible temper brittleness and low temperature tempering brittleness or martensite temper brittleness, etc.Another brittleness occurs in some alloy structural steel, for direct heating in the temperature range of 450 ~ 550 tempering or higher than 600 tempering and slow cooling in 450 ~ 550 interval, has nothing to do with the heat preservation time, and is associated with cooling speed, methods to eliminate this kind of brittleness is heated to above 600 again, rapid cooling can be eliminated, can prevent the occurrence of temper brittleness, the brittleness of the second category of temper brittleness, also known as the reversible temper brittleness, high temperature tempering brittleness or temper brittleness, etc.

Schematic for temper brittleness of structural steels

  Fig. 1 Schematic of temper brittleness of structural steel

  (1) The Charpy impact energy of the first kind of tempered brittle steel parts after quenching has a trough in the first kind of tempered brittle region with the change curve of tempering temperature. The mechanical property index of steel has a different sensitivity to the first kind of tempered brittleness, and it is related to the loading mode.It should be noted that such as the presence of stress concentration, impact or a torsional load of parts, and require greater plasticity and toughness with strength, the emergence of the first kind of tempering brittleness will increase the risk of brittle cracking of parts, so it is a heat treatment defect. Such remedial measures are re-quenching according to the heat treatment process specification, which is generally believed to be due to the decomposition of carbides from martensite, thus reducing the fracture strength of grain boundaries and avoiding the tempering brittle zone. Appropriately increasing the content of silicon in the material can reduce the brittleness of low temperature tempering, which should be seriously considered in the selection of materials.

  (2) The second kind of tempering brittleness is mainly produced in the alloy structure steel containing chromium, nickel, manganese, silicon, and other alloy elements, due to the enrichment of antimony, phosphorus, tin, arsenic, and other impurity elements on the grain boundary, so strengthen the brittleness of the grain boundary caused by tempering brittleness. The characteristics of this type of steel are as follows.

  When the quenched steel is tempered or passed slowly within the brittleness temperature range (500 ~ 650), the tempering brittleness will appear. The longer the stay or holding time is, the brittleness will be obvious.

  As a result, the impact value of the parts at room temperature decreased significantly.

  The brittleness of tempering is related to the cooling rate after tempering, and the brittleness can be suppressed or weakened by rapid cooling.

  This kind of tempering brittleness is reversible, the brittleness can be eliminated if the steel has been tempered at a high temperature and then cooled quickly, and the brittleness will appear again if the steel has been tempered in the range of brittleness temperature.

  Such tempering brittleness will result in brittle fracture of steel along grain boundaries.

  The second type of tempering brittleness of the suppression and prevention measures are as follows.

  In the process of steel smelting, the content of P, Sb, Sn, As, and other harmful impurities in molten steel can be reduced to prevent their segregation at grain boundaries.

  The addition of 0.2% ~ 0.5%Mo or 0.4% ~ 1.0%W to the steel, molybdenum is used to slow down the segregation and diffusion of impurity elements such as P towards the grain boundary, or the selection of steel containing molybdenum or tungsten, both of which reduce the enrichment of impurity elements at the grain boundary by preventing their diffusion.

  High temperature tempering after the end of rapid cooling, or as far as possible to shorten the parts in the brittle temperature residence time and fast cooling after tempering.

  By incomplete quenching or two-phase quenching, fine grains can be obtained to reduce and eliminate the temper brittleness. On the other hand, impurities can be concentrated in the ferrite to avoid segregation toward the grain boundary.

  The austenite grain was refined.

  The tempering brittleness of steel can be eliminated by high temperature deformation heat treatment.

  When the parts are nitriding for a long time, molybdenum steel with low tempering brittleness sensitivity should be selected.The gas nitride of parts is carried out in the range of 500 ~ 550, with a long time (40 ~ 70h) and a thick permeation layer, usually in the range of 0.3 ~ 0.6mm. Nitrogen use with good abrasion resistance, high fatigue strength requirement of precision parts heat treatment process, but it’s important to note that in order to reduce the surface of the parts brittleness, after meeting the requirements of infiltration layer should be back nitrogen treatment (540 ~ 560 x 2 ~ 3 h, ammonia decomposition rate is above 80%), the process is a very important link, otherwise will cause early failure of the parts, directly affect the normal use of spare parts.

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