Discussion: forging cracks, heat treatment cracks and raw material cracks

First of all, the concepts of “raw material cracks” and “forging cracks” need to be determined first.

The cracks that appear after forging should be understood as “forging cracks”, but the main factors leading to forging cracks can be further divided into:

  1. Forging cracks caused by raw material defects;
  2. Forging cracks caused by the improper forging process.

Forging cracks

The macroscopic shape of the crack is firstly distinguished, horizontal generally has nothing to do with the base material, and vertical cracks need to be analyzed in combination with the crack shape and forging process.

If there is decarburization on both sides of the crack, it must be produced during the forging process, as to whether it is caused by the raw material or the forging process, it needs to be analyzed according to the metallographic and technological process.

For the same batch of the same type of workpieces, the forging cracks are basically in one position, extending shallowly under the microscope, with decarburization on both sides.

But the material cracks may not be repeated at the same location, and the depth of the microscope is different.

There are certain rules if you look at and analyze more.

Material cracks are mostly in line with the material longitudinally.

And wrought cracking has two types, one is caused by overheating and overcooking, there is oxidative decarburization near the crack.

The other is crack caused by hitting cold iron, which has lattice damage tearing.

It can be distinguished from metallography.

Therefore, forging and forging raw materials within a certain defect is duty.

Large castings and forgings are often directly from the forging of steel ingots to begin with, there must be a large number of internal steel ingots smelting casting defects.

Obviously, a reasonable forging, all of these  “defects” can be forged together.

Therefore, the reasonableness of the forging process is the main reason to determine whether the forging will crack.

However, in a stable forging process, if there is a clear requirement for controlling the level of raw material defects before forging, the cracking phenomenon that occurs when the level of raw material defects exceeds the requirement and is forged under the original forging process, which can be regarded as “forging cracks caused by raw material defects”.

Specific problems of the cracks need to be analyzed specifically, combined with the process analysis, including the heating process with no protective atmosphere should be considered, forging is supposed to forge the cracks in the raw material together.

The oxidized layer is usually dense with gray color, the dirt caused by the sample making process is very loose with dark color, which can be distinguished at first glance under high magnification, and they can be distinguished with the energy spectrum directly when you really can’t see the difference.

crack caused by hitting cold iron

Forging cracks

Forging cracks are generally formed at high temperatures when forging is deformed because the crack expands and comes into contact with air, so when observed under a 100X or 500X microscope, the crack is filled with oxide and the 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 and without fine directionality.

In addition to the above typical patterns, some forging cracks are sometimes finer and the crack is not fully decarburized but semi-decarburized around.

Heat treatment crack

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 steels, 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.

It produces cracks distributed along the coarse grain boundaries; 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, but the inside of the crack and its tail are filled with oxide.

Sometimes due to the failure of the high-temperature equipment, the temperature is very high, which results in the extremely coarse structure of the parts, and the cracks are distributed along the coarse grain boundaries.

Heat treatment crack

Reasons for forging cracks and heat treatment cracks

Reasons for forging cracks

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 cavities, white spots and interlayers, they may become the reasons to produce cracks.

In addition, due to poor forging process or improper operation, such as overheating, overburning or too low final forging temperature and too fast cooling rate after forging etc., the forgings may also produce cracks.

Reasons for heat treatment cracks

Quenching cracks are macroscopic cracks, which are 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 rate etc.

On the one hand, increasing the quenching internal stress will cause the formed quenching microcracks to expand and form macroscopic quenching cracks; on the other hand, because the sensitivity of the microcracks is increased, the number of microcracks is increased and the number of microcracks is reduced, which reduces the brittle fracture resistance of the steel to Sk, thus increasing the likelihood of quench crack formation.

Factors affecting quenching

There are many factors that affect the quenching crack, following are several situations in production:

  1. Quenching due to existing defects in the raw material: If defects such as cracks or inclusions in the surface and interior of the raw material are not detected prior to quenching, quench cracks may form.
  2. Cracks due to inclusions: if the inclusions within the parts are serious, or itself has hidden cracks due to heavy inclusions, which will be likely to develop when quenched.
  3. Quenching due to poor primary tissue.
  4. Quenching cracks caused by improper quenching temperature: Inappropriate quenching temperature causes quenching of parts.

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 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 quenching according to the normal process, it is equivalent to increasing the quenching temperature of the steel, which is likely to cause the parts to overheat and grain growth, resulting in increased stress during quenching and quenching cracking.

  1. Quenching cracks caused by improper quenching cooling: due to improper cooling during quenching, quenching cracks will also occur in parts.
  2. 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 simple part or a place where stress is not concentrated, it will also cause cracks during quenching, or early damage during the process.

  1. The influence of the shape of parts on 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 generated due to stress concentration during quenching.
  2. Cracking due to untimely tempering: If not tempered in time after quenching, cracks may occur due to excessive quenching residual stress.

In summary

Applying metallographic methods for defect analysis is a more complex task, as part failure is sometimes caused by multiple factors.

Therefore, in practice, you should do a multifaceted investigation to master the facts and analyze them from various aspects to ensure the accuracy of defect analysis.

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