What are the main materials where thermal cracking occurs

Landlord: Hello!

It's a coincidence, there was just a cold crack problem in front, and here is another hot crack, so let's just take a look together!

First of all, let's first understand the types of thermal cracks: mainly crystalline cracks, high-temperature liquefaction cracks, and multi-variation cracks;

Secondly, let's understand the mechanism of thermal cracking: there is too much content, and I will briefly describe it here as the stress + stress weak boundary;

Again, let's understand the influencing factors: mainly the influence of chemical components such as S, P, C, SI, MN, including such as low melting point eutectics, inclusions, segregation, etc., note that when Si is less, it is conducive to eliminating hot cracks, but higher than the opposite, it is easy to form low melting point silicate inclusions, and Mn can play a role in desulfurization, so it has an anti-cracking effect.

Finally, let's take a look at the materials, in fact, low carbon steel, low alloy steel, stainless steel, nickel-based alloys all have a tendency to crack hot, especially the harmful elements S, P, C, so the amount of these elements directly determines the tendency to crack hot, about the thermal crack sensitivity index and its calculation in my other should have, I will not repeat it here, for Ni and S to form Ni3S2 low melting point eutectic melting point melting point is only 645, the most easy to form hot cracks in engineering, especially arc pit cracks are very common, Therefore, for chromium-nickel stainless steel, it is hoped that there is a certain amount of ferrite to improve its crack resistance and corrosion resistance, and FN is usually required to be 4-8.

All right! That's all for a brief explanation of thermal cracking, and I hope you can deepen your understanding of it! Thank you!

Hot cracking occurs mainly on materials with high sulfur content.

When the weld crystallizes, impurities and low-melting eutectic segregation at the grain boundaries. Since they have a lower melting point than weld metals, they exist as a liquid interlayer during crystallization. When subjected to a certain amount of tensile internal stress, the liquid interlayer is pulled apart and a thermal crack is formed.

It can be seen that tensile stress is the external cause of hot cracking, and the low melting point eutectic on the grain boundary is the internal cause of the hot crack, and the tensile stress causes the hot crack through the low melting point eutectic on the grain boundary.

The preventive measures are:

1) Reduce the sulfur content of the base metal or welding wire.

2) Reduce the carbon content of the weld.

3) Increase the manganese content of the welding wire.

4) Add deterioration.

5} Formation of biphasic structure.

6) Choose a reasonable forming factor.

7) Choose a reasonable welding sequence and welding direction.

8) Preheating before welding and slow cooling after welding.

In welding engineering, cracks are unavoidable, and cracks are the most dangerous defects and must be paid great attention. The following is a detailed description of the causes of hot cracks and cold cracks.

1. Cold cracks.

1) Cold cracks in the weld When the weld is cast iron, this crack is more likely to appear.

2) Cold cracks in the heat-affected zone: Most of these cracks occur in the heat-affected zone with more cementite and martensite, and in some cases may also occur in the heat-affected zone slightly away from the fusion line.

2. Thermal cracks.

When the cold welding of mild steel and nickel-based cast iron electrodes is used, the weld is more prone to crystalline cracks that belong to hot cracks.

1) Characteristics of thermal cracks.

Thermal cracks often occur in the weld area, which is called crystalline crack in the process of weld crystallization, and also occurs in the heat-affected zone, when heated to the superheating temperature, the intergranular low melting point impurities melt and produce cracks, which are called liquefaction cracks.

Features: Cracking along the grain boundary (so it is also called intergranular crack), and there is oxidation color on the surface of the fracture.

2) Causes of thermal cracks:

There is a liquid intergranular layer between the grains.

Welds: presence of low melting point impurities segregation } Formation of a liquid interlayer.

Heat-affected zone: There are low-melting impurities in the grain boundaries of the superheated zone.

There is a welding tensile stress.

3) Thermal crack prevention measures:

Metallurgical factors. Thermal cracking.

Tensile stress limits the low melting point impurities such as S and P content in steel and welding consumables.

Control the welding specification and appropriately improve the weld forming coefficient (that is, the ratio of the width of the weld bead to the calculated thickness) The forming coefficient of the jujube weld is too small, which is easy to form centerline segregation and hot cracks.

Adjust the chemical composition of the weld to avoid low melting point eutectics; Reduce the crystallization temperature range, improve the weld structure, refine the weld grain, improve plasticity, and reduce segregation.

Reduces welding tensile stress.

operationally fill the arc pit.

Cold cracks(1) Morphology and characteristics of cold cracks.

Cold crack morphology { Crack under the weld bead: A weld cold crack that forms in the heat-affected zone under the weld bead, often developing parallel to the fusion line.

Weld Finger Crack: A cold crack that forms along a weld site where stress is concentrated, propagating within the influence of heat.

Root Cracks: Cold cracks that form along the root of a weld where stress is concentrated, and develop into a weld or heat influence.

Figure 5-2-17 Cold crack in welding.

a-Crack under the weld bead; b-Weld toe crack; c-Root cracks.

Features: no branching, penetrating cracking, no oxidation color on the fracture surface.

The most predominant and common type of cold crack is delayed crack (i.e., a crack that occurs after a delay of some time after welding--- because hydrogen is the most active trigger, and it takes a certain amount of time for hydrogen to diffuse, aggregate, and induce cracks in the metal).

2) Cause of delayed cracks.

There is a hardened structure in the welded joint, and the performance is embrittle.

The high content of diffused hydrogen embrittles the joint properties and collects a large number of hydrogen molecules at the welding defects, causing very large local pressures. (Hydrogen is the most active factor that induces delayed cracking, so some people call delayed cracking also hydrogen-induced cracking).

There is a large tensile stress in welding.

3) Measures to prevent delayed cracking.

Alkaline electrodes are selected to reduce the hydrogen content in the weld metal and improve the plasticity of the weld metal.

Reduce hydrogen** jujube welding consumables should be dried, and joints should be clean (no oil, no rust, no water).

Avoid preheating before welding and slow cooling after welding (which can reduce the cooling rate after welding).

To reduce welding stress, jujube adopts reasonable process specifications, post-welding heat treatment, etc.

Immediately after welding, hydrogen deactivation treatment is carried out (i.e., heated to 250, heat preservation 2 6 or so, so that the diffused hydrogen in the weld metal escapes from the metal surface).

Summary. A crack is a type of grain crack that develops at high temperatures in the weld metal and in the heat-affected zone of the weld. Cold cracks are cracks that form due to the embrittlement of a material at a temperature near room temperature.

Both preheating and post-weld heat treatment control cold cracking, one to control the formation of brittle hard tissue and the other to eliminate the amount of diffused hydrogen.

Hello, I have seen your question and am sorting out the answer, please wait a while Crack is a kind of grain crack that occurs in the weld metal and the weld heat-affected zone at high temperatures. Cold cracks are cracks that are formed due to the embrittlement of the material at temperatures near room temperature. Both preheating and post-weld heat treatment are known to control cold cracking, one to control the formation of brittle hard tissues, and the other to eliminate the content of diffused hydrogen.

Summary. Dear, Hello, local overheating can produce thermal cracks.

Dear, Hello, local overheating can produce thermal cracks.

Dear, Hello, Thermal Cracking: Cracking along the grain, generally occurs near the weld or in the weld area. It has an oxidized color and a five-metal luster.

It is mainly divided into three categories: crystalline cracks, high-temperature liquefaction cracks and multi-variation cracks. Thermal crack refers to the crack produced at high temperature, also known as high temperature crack or crystalline crack, which usually occurs inside the weld first stroke, and sometimes tung must also appear in the heat-affected zone, which is manifested in the form of: longitudinal crack, transverse local crack.

Answer] :d This question examines the welding process procedures and techniques. Ways to prevent reheat cracks:

1) Preheating: The preheating temperature is 200 450°C. If the preheating temperature can be lowered appropriately after welding. For example, after 18mnmonb steel welding, heat treatment at 180°C is carried out immediately for 2h, and the preheating temperature can be reduced to 180°C.

2) The application of low-strength weld makes the weld strength low in the mold hall of the base metal to increase its plastic deformation capacity.

3) Reduce welding stress, reasonably arrange the welding sequence, reduce the height of the afterglow code, avoid undercut and root failure to weld through and other defects to reduce welding stress.

The shape characteristics of the thermal crack of the casting are ().

a.The shape is zigzag.

b.The gap is wider.

c.There is an oxidized color in the seam.

d.The cracks are fine.

e.There is slag inclusion in the seam.

f.Cracks penetrate the grains.

g.The cracks are straighter.

Correct Answer: Twists and turns in shape; The gap is wider. There is an oxidized color in the seam.