Forging Crack And Heat Treatment Crack Morphology
Forging cracks are generally formed at high temperatures. During forging deformation, the cracks expand and come into contact with air. Therefore, when observed under a 100X or 500X microscope, the cracks are filled with oxide scale;
And the two sides are decarburized, the structure is ferrite, and its morphological characteristics are that the cracks are relatively thick and generally exist in multiple forms, without fine tips, relatively round and pure, without detailed directionality;
In addition to the above typical shapes, sometimes some forging cracks are relatively fine. The area around the crack is not completely decarburized but semi-decarburized.
The cracks produced during the quenching heating process and the cracks formed during the forging heating process are obviously different in nature and shape. For structural steel, the heat treatment temperature is generally much lower than the forging temperature, even for high-speed steel and high-alloy steel, the heating and holding time is much shorter than the forging temperature.
Due to the high heating temperature of heat treatment, too long holding time or rapid heating, early cracking will occur during the heating process. Cracks distributed along the coarse grain boundaries are generated; there is a slight decarburization structure on both sides of the crack, and the heating speed of the parts is too fast, and early cracks will also occur. There is no obvious decarburization on both sides of the crack.
However, the inside of the crack and its tail are filled with oxide scale. Sometimes due to the failure of the high-temperature equipment, the temperature is very high, resulting in the extremely coarse structure of the parts, and the cracks are distributed along the coarse grain boundaries.
Common Defects Of Structural Steel:
1 Forging Defects
(1) Overheating and overburning: The main feature is coarse grains and obvious Widmanstatten structure. The occurrence of over-burning indicates that the heating temperature is high, the fracture grain is coarse, uneven, there is no metallic luster, and there is oxidation and decarburization around the grain boundary. To
(2) Forging cracks: often occur in areas where the structure is coarse, where stress is concentrated or where alloy elements are segregated, and the inside of the crack is often full of oxide scale. High forging temperature or low terminal temperature are prone to cracks. Another type of crack is formed after forging and cooling by spraying water.
(3) Folding: Surface defects are caused by punching, cutting, blade wear, rough forging and other reasons. In the subsequent forging, defects such as surface oxide scale are rolled into the forging body to form creases. When observed on a microscope, it can be found that there is obvious decarburization around the fold.
2 Heat Treatment defects
(1) Quenching and cracking: It is characterized by being strong and straight, with a cross-crystalline distribution, a wide starting point, and a slender and winding tail. Such cracks mostly occur after martensite transformation, so the microstructure around the cracks is not significantly different from other areas, and there is no decarburization.
(2) Overheating: The microstructure is coarse. If it is slightly overheated, secondary quenching can be used to save it.
(3) Overburning: In addition to the coarse grains, some grains have tended to melt, and the grain boundaries are extremely coarse.
(4) Soft point: The microstructure includes massive or reticulated troostite and undissolved ferrite. Insufficient heating, insufficient holding time, and uneven cooling will produce soft spots.
Causes of forging cracks and heat treatment cracks
1 Forging Crack
During the forging process of steel, due to the surface and internal defects of the steel, such as hairline, blisters, cracks, inclusions, subcutaneous bubbles, shrinkage pores, white spots and interlayers, these may be the cause of forging cracks.
In addition, due to poor forging process or improper operation, such as overheating, over-sintering or too low final forging temperature, too fast cooling rate after forging, etc., the forgings may also crack.
2 Heat Treatment Cracks
Quenching cracks are macroscopic cracks, mainly caused by macroscopic stress. In the actual production process, steel workpieces are often due to unreasonable structural design, improper steel selection, incorrect quenching temperature control, and inappropriate quenching cooling speed;
On the one hand, increasing the quenching internal stress will cause the formed quenching micro cracks to expand and form macro quenching cracks;
On the other hand, as the sensitivity of microcracks is increased, the number of microcracks is increased, and the brittle fracture resistance Sk of the steel is reduced, thereby increasing the possibility of quenching crack formation.
There are many factors that affect quenching and cracking.
Here only a few situations that are often encountered in production are introduced:
- Quenching cracks caused by defects in the raw materials: If there are defects such as cracks or inclusions on the surface and inside of the raw materials, which are not found before quenching, quenching cracks may form. To
- Cracks caused by inclusions: If there are serious inclusions inside the part, or there are cracks hidden due to serious inclusions, cracks may occur during quenching.
- Quenching and cracking caused by poor original organization
- Quenching cracks caused by improper quenching temperature, and parts quenching cracks caused by improper quenching temperature. Generally, there are two situations:
(1) The temperature indicated by the instrument is lower than the actual temperature of the furnace, which makes the quenching temperature too high, causing overheating of the quenching and cracking of the workpiece. Any metallographic structure cracked by overheating and quenching has coarse grains and coarse martensite.
(2) The actual carbon content of steel parts is higher than the content specified by the steel grade. If quenched according to the normal quenching process of the original grade, the quenching temperature of the steel will be increased, which will easily cause the parts to overheat and the crystal grains to grow. The increase in stress causes quenching cracks.
- Quenching cracks caused by improper quenching and cooling. Due to improper cooling during quenching, quenching cracks will also occur in parts.
- Quenching cracks caused by machining defects. Due to poor machining, thick and deep knife marks are left on the surface of the parts. Even if it is a very simple part or a place where stress is not concentrated, it will also cause cracking during quenching, or Early damage occurred during service.
- The influence of the shape of the parts on the quenching cracks, the geometric shape of the parts is unreasonable, or the thickness of the transition zone of the cross-section is quite different, and cracks are easily caused by stress concentration during quenching.
- Cracks caused by not timely tempering, and not timely tempering after quenching, may cause cracks due to excessive quenching residual stress.
In short, the application of metallographic methods for defect analysis is a relatively complicated task. Because the failure of parts is sometimes caused by multiple factors, in actual work, we should conduct various investigations, grasp the facts, and carry out many aspects. Analysis to ensure the accuracy of defect analysis.
Heat treatment crack generation mechanism and grinding crack generation mechanism
Heat Treatment Crack Generation Mechanism:
The cracking of the workpiece after quenching is caused by internal stress. Internal stress is divided into thermal stress and phase change stress.
(1) When the workpiece is heated or cooled, the stress caused by the inconsistency of thermal expansion or contraction due to the temperature difference in different parts is called thermal stress.
(2) When the quenched workpiece is heated, ferrite and cementite transform into austenite, and when cooled, it transforms from austenite into martensite. Due to the different specific volumes of different tissues, volume changes must occur during heating and cooling.
In the process of heat treatment, the stress caused by the temperature difference between the surface and the core of the workpiece that causes the transformation of each part of the structure to not proceed at the same time is called phase transformation stress. During quenching and cooling, if the internal stress in the workpiece exceeds the yield point of the material, plastic deformation may occur. If the internal stress is greater than the tensile strength of the material, the workpiece will crack. To
Mechanism Of Grinding Cracks:
The high grinding temperature easily burns the surface of the workpiece, so that the surface of the quenched steel is annealed and the hardness is reduced. Even if secondary quenching may occur due to the pouring of cutting fluid, tensile stress and micro-cracks will be generated on the surface of the workpiece, reducing the surface quality and service life of the part.
Causes Of Heat Treatment Cracks Of Parts
The parts will produce large internal stresses (structural stress and thermal stress) during the heat treatment process. When these stresses exceed the yield strength of the steel, they will cause deformation of the parts; when the stress is greater and exceeds the tensile strength of the steel, then It will cause the parts to crack.
There are two kinds of stress acting on the part: compressive stress and tensile stress
The tensile stress formed during quenching is the main cause of quenching cracks. But when the plasticity of the steel is high, even if there is a large tensile stress, it will not cause the parts to crack.
For example, stress relief annealing without structural transformation, austempering to obtain more retained austenite, etc. Only when the stress is high and the structure with high hardness and brittleness is high, it is easy to cause the parts to crack.
Therefore, two conditions must exist for the formation of quenching cracks:
One is that it has a brittle structure; the other is that the tensile stress exceeds the tensile strength of the steel at this time (of course, other conditions can also promote part cracks, such as raw material defects, defects caused by improper design and machining, etc.).
About The Type Of Crack
There are various ways to classify cracks. According to the direction of the crack, there are longitudinal cracks, transverse cracks, arc cracks and network cracks (also called cracks), etc.; according to the location of the cracks, there are surface cracks (or surface cracks) and internal cracks;
According to the cracks occurring in different processes, there are forging cracks, welding cracks, quenching cracks, tempering cracks, cold treatment cracks, pickling cracks and grinding cracks. Quenching cracks are the most common parts in the heat treatment process.
Crack Resolution Method
How to distinguish whether it is a quenching crack, a tempering crack, a forging crack or a grinding crack is very important, so that it is convenient to accurately find the process in which the crack occurs, and is beneficial to analyze the cause of the crack.
1.Note the difference between quenching cracks and grinding cracks. For cracks not found during quenching but found after grinding, it is necessary to distinguish between quenching cracks and grinding cracks. It is easier when the crack is not attached to contaminants; at this time, pay attention to the shape of the crack, especially the direction of crack development. The grinding crack is perpendicular to the grinding direction, in the form of parallel lines, or in the shape of a tortoise shell.
The depth of the grinding cracks is shallow, while the quenching cracks are generally deeper and larger, and have nothing to do with the grinding side. Most of the cracks are straight cut.
2.Pay attention to the places where cracks occur, sharp concave and convex corners, hole edges, markings, stamping places and surface defects caused by machining. The cracks that occur in these places are mostly quenching cracks.
3.Distinguish by observing the cracked section of the part, distinguish whether it is a quenching crack or a forging crack before quenching or a crack caused by other conditions. If the crack section is white or dark white or light red (water rust caused by water quenching), it can be judged to be a quenching crack; if the crack section is dark brown, even if there is oxygen skin, it is not a quenching crack, and it is before quenching The existing cracks are the cracks formed by the forging or rolling of the parts, and these cracks will be enlarged by quenching.
Because quenching cracks are basically formed below the MS point, the section will not be oxidized.
4.In the microstructure, the quenching crack fractures along the grain boundary. If it does not fracture along the grain boundary but fractures along the grain, it is a fatigue crack.
5.If there is a decarburized layer around the crack, it is not a quenching crack, but a crack that exists before quenching, because the quenching crack is during quenching and cooling.Will never decarbonize
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