Discrimination of Typical Magnetic Marks of Forged Steel Crankshaft

Crankshaft is one of the important parts of diesel engine.

In order to ensure the quality of the crankshaft, 100% flaw detection is carried out after the finished crankshaft is finished.

In general, major engine manufacturers have their own crankshaft flaw detection standards. During the flaw detection process, various magnetic marks with various forms, complex causes and difficult solutions often occur.

The type of magnetic marks is usually judged by the experience of the flaw detector, especially the judgment of grinding cracks.

Generally, for the convenience of flaw detection observation, crankshaft manufacturers carry out magnetic particle flaw detection after grinding. Once cracks occur, cold and hot processing are controversial.

Through many years of production practice and analysis, several typical magnetic flaw detection marks of induction hardened crankshaft are summarized to provide a basis for more scientific on-site judgment.

1. Process route of forged steel crankshaft

Blanking → forging → normalizing, quenching and tempering → rough machining → stress relief → semi finishing → induction quenching and tempering → finishing → inspection and warehousing

2. Detection method of crankshaft surface defects

At present, the common method to detect the surface defects of crankshaft is to use fluorescent magnetic particle testing machine or visual inspection.

3. Surface defect magnetic trace

The surface defects are those visible to the naked eye under good lighting when the magnetic suspension is cleaned after magnetic particle flaw detection.

The magnetic marks of such defects are surface defect magnetic marks.

Common surface defects of forged steel crankshaft include: raw material and forging cracks, heat treatment cracks, grinding cracks and exposed non-metallic inclusions.

(1) Appearance and identification of raw material cracks and forging fold cracks

Raw material cracks and forging fold cracks are generally relatively small on the forging blank surface, which is difficult to find without careful observation and inspection, but these cracks will develop after processing and induction hardening.

If it is a small part, sometimes under the action of internal stress, it will be seriously divided into two parts.

The cracks of raw materials are generally parallel to the axis, extending straightly and intermittently, as shown in Fig. 1.

Discrimination of Typical Magnetic Marks of Forged Steel Crankshaft 1

Fig. 1 Raw Material Cracks

Forging folding is the interlayer caused by improper operation during forging. Its shape and position are uncertain.

After quenching and stretching, the crack is relatively large, as shown in Fig. 2. The involved oxide skin can sometimes be seen at some main openings.

Discrimination of Typical Magnetic Marks of Forged Steel Crankshaft 2

Fig. 2 Forged fold

See Fig. 3 for forgings with cracks.

A rough analysis might be:

① Because of overburn.

② As the soluble metal penetrates into the base metal (such as copper).

③ Stress corrosion cracking.

④ The forging surface is severely decarburized.

This can be further distinguished from the process investigation and organization analysis.

For example, when the steel is heated after the steel is heated, or both are heated, or the copper content in the steel is too high, it may be copper brittleness.

From the view of microstructure, the copper embrittlement cracks at the grain boundary.

In addition to the cracks, bright copper mesh can be found, while only oxides can be found at the pure overburned grain boundary.

Stress corrosion cracking occurs after acid pickling. When observed at high magnification, the crack extends in a dendritic shape.

When the forging is severely decarburized, a thicker decarburized layer can be observed on the test piece.

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Fig. 3 Forging Cracking

For raw material cracks and forging cracks, as long as we take samples perpendicular to the cracks, we can find decarburization on both sides of the cracks during metallographic observation, and sometimes oxides are sandwiched between them.

(2) Appearance and Identification of Quenching Cracks

Crankshaft quenching cracks generally occur in places with sudden changes in size, thin effective thickness or poor surface roughness.

There are steps, ends, sharp corners, keyways, holes, oil passages and other structures in the quenching area.

Induction quenching will cause the concentration of induction current in this part, resulting in local overheating and quenching cracks caused by too deep hardening layer.

Quenching cracks generally have two manifestations.

Quenching cracks that appear on a smooth cylindrical surface or near a boss with a thin effective thickness are distributed circumferentially with relatively large dimensions, as shown in Fig. 4.

The other crack is oil hole crack, as shown in Fig. 5 and Fig. 6.

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Fig. 4 Cracks on top dead center boss of connecting rod journal

Discrimination of Typical Magnetic Marks of Forged Steel Crankshaft 5

Fig. 5 Radial cracks at oil orifice

Discrimination of Typical Magnetic Marks of Forged Steel Crankshaft 6

Fig. 6 Transverse small cracks on inner wall of oil hole

In addition to radial cracks near the crankshaft oil hole, sometimes small transverse cracks appear in the area of 3~8mm on the inner wall below the oil hole, or “C” shaped cracks also appear on the journal surface near the hole, which generally appears on the inclined oil passage, 10~20mm from the hole edge in an arc shape.

This type of crack is due to the different thicknesses from the inner wall of the inclined oil passage to the surface of the journal.

It is very easy to harden at the thinnest place, and the depth of the hardened layer is relatively deep.

When the quenching process is not appropriate, small transverse cracks in the hole will appear at the place where the oil passage hole wall is thin and hardened.

If such cracks extend to the surface in the form of flakes and penetrate the surface, a “C” shaped crack at the orifice will be formed.

The quenching crack is observed under the metallographic microscope, which is obviously different from the raw material crack and forging crack.

There is no decarburization and oxide in the crack, and the tail end is thin.

If the heating temperature is too high, the quenching cracks are distributed along the grain, and the superheat characteristics such as coarse acicular martensite can be observed;

Quenching cracks caused by too fast cooling in the martensite transformation zone are usually transgranular, and the cracks are straight, with strong lines, and there are no branching small cracks around.

The metallographic structure near the main crack is generally characterized by fine tempered martensite.

(3) Appearance and Identification of Grinding Cracks

After induction hardening of crankshaft, the surface hardness is high and the internal stress is large. When the grinding parameters are not correct, grinding cracks will appear.

The process of grinding cracks is actually a quenching process.

During high-speed grinding, the temperature of the local area where the grinding wheel contacts the workpiece is above the austenitizing temperature.

When the cutting fluid is sprayed on the grinding wheel and the workpiece, it is equivalent to another quenching.

When the material contains some trace alloy elements that increase the tendency of quenching cracks, the probability of grinding cracks will be increased.

Grinding cracks occur on the ground smooth surface. Typical grinding cracks on the journal are:

Cracking cracks (in the shape of Japanese, mouth and well), as shown in Fig. 7, are single linear cracks, multiple parallel point and strip cracks or a pile of point and strip cracks, as shown in Fig. 8.

A single linear crack or multiple parallel point cracks are distributed in the axial direction, perpendicular to the grinding direction.

It is generally radial on the side boss, as shown in Fig. 9.

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Fig. 7 Cracking

Discrimination of Typical Magnetic Marks of Forged Steel Crankshaft 8

Fig. 8 Multiple Parallel Point and Strip Cracks or a Pile of Point and Strip Cracks

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Fig. 9 Lateral radial cracks

If the sample is perpendicular to this type of crack, the secondary quenching structure can be seen under the metallographic microscope.

The metallographic characteristics of the secondary quenching layer from the outside to the inside are white bright layer, black gray tempering layer (troostite), and induction hardening layer (tempered martensite).

It can be seen from the size of the hardness indentation of the white bright layer, and its hardness is particularly high, as shown in Figure 10 and Fig. 11.

Sometimes only the tempered troostite layer can be seen, but the white bright layer of secondary quenching can not be seen.

The secondary tempering layer is very thin, which requires higher sample preparation requirements for metallographic samples.

If the sample preparation is not good, the white bright layer may not be seen.

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Fig. 10 Metallographic Structure of Secondary Quenching Layer for Grinding Cracks

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Fig. 11 Comparison Diagram of Hardness Indentation of Secondary Quenching Layer for Grinding Cracks

(4) Appearance and identification of exposed non-metallic inclusions

The inclusions are generally divided into metal inclusions and non-metallic inclusions.

Metal inclusions are generally external, which can be completely avoided by strengthening management and strict operation.

Non metallic inclusions are the reaction products of gases in steel, deoxidizer and alloy elements during smelting, as well as refractory fragments mixed in steel.

During smelting, it is mainly through the full boiling of liquid steel and the full stabilization in the steel ladle to fully float the inclusions and discharge them into the slag.

The position of non-metallic inclusions is not fixed, and there are single or intermittent inclusions.

Since non-metallic materials are nonmagnetic, their existence has destroyed the continuity of materials.

If inclusions have been exposed or the length near the surface is relatively long, magnetic particle aggregation – magnetic marks will appear in magnetic particle flaw detection;

The distance between non-metallic inclusions and the surface is different, and their manifestations are also different.

The closer to the surface, the more obvious the magnetic trace display.

For this reason, sometimes the magnetic traces of inclusions are intermittent.

After the crankshaft is forged, non-metallic inclusions are also generally distributed along the axial direction, and the lines of its magnetic traces appear soft, and the tail end is bald.

When the inclusion is exposed after processing, it is an open defect. See Fig. 12.

Discrimination of Typical Magnetic Marks of Forged Steel Crankshaft 12

Fig. 12 Single open inclusion and enlarged morphology

The sample is taken perpendicular to the crack and observed under the metallographic microscope.

The depth is not deep and the bottom is round, as shown in Fig. 13.

Discrimination of Typical Magnetic Marks of Forged Steel Crankshaft 13

Fig. 13 Cross section without corrosion

4. Appearance and identification of non surface defect magnetic traces

After magnetic particle flaw detection, when wiping the magnetic suspension, observe it with naked eyes (normal vision) under good lighting.

If there is no defect, it is a non surface defect magnetic mark.

(1) Forging streamline

During the forging process of forged parts, the metal flows in a certain direction.

If dissected, the forging streamline can be seen through macro observation after corrosion, as shown in Fig. 14.

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Fig. 14 Macro picture after 5% nitric acid alcohol corrosion

During flaw detection under normal process specification, generally no forging streamline magnetic trace or very weak magnetic trace is displayed, as shown in Fig. 15.

Only when the magnetic field is too strong or accompanied by segregation and a certain amount of inclusions can it be clearly displayed.

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Fig. 15 Front end streamline of crankshaft

(2) Segregation and inclusion

The non-uniformity of the chemical composition of steel grades is called segregation.

According to the different causes and manifestations of segregation, it is generally divided into dendritic segregation, square segregation, and point segregation.

Generally, there are many inclusions in these segregations, which are unavoidable in ingots, especially in medium carbon chromium molybdenum and chromium nickel molybdenum steel forgings.

Because the addition of alloy elements will often reduce the fluidity of molten steel, alloy steel is more difficult to remove than carbon steel non-metallic inclusions, and more likely to cause segregation or inclusions.

For alloy steel crankshaft, metal flows from the center to the parting surface during the processing of the crankshaft, so segregation (ribbon) and inclusions are generally relatively serious at the trimming and tend to the surface.

Even if these inclusions are not exposed at the cutting edge, as long as they are close to the surface and have a certain length, streamline magnetic marks will appear during flaw detection, as shown in Fig. 16.

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Fig. 16 Streamline at Trimming of Stereo Parting Crankshaft and Strip Segregation on the Surface after Macroetching with 5% Nitric Acid

Due to the high hardenability of alloy steel, the steel with such metallurgical defects is easy to produce structural inhomogeneity (banded structure) during cooling.

In addition, the thermal conductivity of alloy steel is relatively poor, which will increase the residual stress in the steel.

Grinding cracks may occur in these areas if the process is slightly improper during grinding.

5. Conclusion

How to correctly judge all kinds of magnetic marks in crankshaft flaw detection requires long-term field practice of flaw detection workers in addition to unity of understanding.

At present, each engine factory has its own crankshaft flaw detection standard.

For the influence of non surface defect magnetic marks on crankshaft fillet and journal on crankshaft fatigue performance, various professional manufacturers have different understandings. This work needs to be further explored by peers.

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