The crankshaft is a crucial component of a diesel engine. To guarantee its quality, 100% inspection is carried out after manufacturing.
Typically, major engine manufacturers have their own set of standards for crankshaft inspection. During this process, several magnetic brands may emerge with different formats, complex origins and challenging solutions.
The classification of magnetic marks is typically determined by the inspector's experience, especially when it comes to identifying grinding cracks.
To facilitate observation during inspection, crankshaft manufacturers often perform magnetic particle inspection after grinding. The treatment of cracks, whether by cold or hot processing, continues to be a subject of debate.
Through years of production and analysis, several magnetic inspection marks typical of induction hardened crankshafts have been identified, providing a basis for more informed on-site judgment.
1. Process route of forged steel crankshaft
Cutting → forging → normalizing, quenching and tempering → rough machining → stress relieving → semi-finishing → induction quenching and tempering → finishing → inspection and storage
2. Crankshaft surface defect detection method
Currently, the most common methods for detecting surface defects on crankshafts are using a fluorescent magnetic particle testing machine or performing a visual inspection.
3. Magnetic trace of surface defect
Surface defects are imperfections that can be seen with the naked eye under good lighting conditions after cleaning the magnetic suspension after magnetic particle inspection. These are called magnetic surface defect marks.
Some common surface defects on forged steel crankshafts 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 bend cracks are typically small and located on the surface of the forging blank, making them difficult to detect without close examination. However, these cracks may worsen after processing and induction hardening.
In the case of smaller parts, internal stress can cause these cracks to seriously split the part in two.
The raw material cracks are generally parallel to the axis, extending straight and intermittently, as shown in Figure 1.
Fig. 1 Cracks in the raw material
Forging bending is an intermediate layer that results from improper operation during forging. Its shape and location are unpredictable.
After quenching and tempering, the crack becomes relatively large, as shown in Figure 2. In some cases, the involved oxide layer can be seen in the main openings.
Fig. 2 Forged bend
See Fig. 3 for forgings with cracks.
A rough analysis might be:
① Because of excessive burning.
② As the soluble metal penetrates the base metal (such as copper).
③ Stress corrosion cracking.
④ The forging surface is severely decarburized.
These fissures can be further differentiated through process investigation and organizational analysis.
For example, overheating of steel or the presence of a high copper content in steel can cause copper to become brittle. From a microstructural point of view, copper brittle cracks occur at the grain boundary.
In addition to the cracks, a bright copper mesh can be seen, while only oxides are present at the pure and burnt grain boundary.
Stress corrosion cracking can occur after acid pickling. When viewed under high magnification, the fissure extends in a dendritic pattern.
If the forging has been severely decarburized, a thicker layer of decarburization may be observed on the test piece.
Fig. 3 Cracks in Forging
For raw material cracks and forging cracks, during metallographic observation, if samples are taken perpendicular to the cracks, decarburization can be observed on both sides of the cracks. In some cases, oxides may be present in the medium.
(2) Appearance and identification of tempering cracks
Quenching cracks in crankshafts generally occur in areas with sudden size changes, thin effective thickness, or poor surface roughness. These areas may include steps, edges, sharp corners, keyways, holes, oil passages and other structures in the quench region.
Induction hardening can cause a concentration of induction current in these parts, leading to local overheating and crack extinction due to a deep hardening layer.
Temper cracks generally have two forms.
A form of crack tempering can be found on a smooth cylindrical surface or near a protrusion with a thin effective thickness. These cracks are distributed circumferentially and have relatively large dimensions, as shown in Figure 4.
The other type of crack is the oil crack, as shown in Figures 5 and 6.
Fig. 4 Cracks at the top dead center of the connecting rod journal
Fig. 5 Radial cracks in the oil hole
Fig. 6 Small transverse cracks on the inner wall of the oil hole
In addition to radial cracks near the oil hole in the crankshaft, small transverse cracks can sometimes appear in an area 3 to 8 mm below the inner wall of the oil hole. Alternatively, “C” shaped cracks can also be found on the surface of the journal near the bore, usually appearing as an arc 10 to 20 mm from the edge of the bore along an inclined oil passage.
These cracks are the result of the different thicknesses between the inner wall of the inclined oil passage and the surface of the journal. The thinner area is more prone to hardening, leading to a relatively deep hardened layer.
If the quenching process is not carried out correctly, small transverse cracks may appear on the wall of the oil passage hole, where it is thin and hardened. If these cracks extend to the surface in the form of flakes and penetrate the surface, a “C” shaped crack may form in the hole.
Quenching cracks are easily distinguishable from raw material cracks and forging cracks under a metallographic microscope. They have no decarburization or oxide and have a thin tail.
If the heating temperature is too high, quenching cracks will be distributed throughout the grain and exhibit overheating characteristics such as coarse acicular martensite. Quenching cracks caused by rapid cooling in the martensite transformation zone are typically transgranular, with straight cracks and strong lines, and no small branching cracks around them.
The metallographic structure near the main crack is typically composed of finely tempered martensite.
(3) Appearance and Identification of Grinding Cracks
After induction hardening, the crankshaft surface has high hardness and high internal stress. If the grinding parameters are incorrect, grinding cracks may occur.
The crack grinding process is similar to the tempering process.
During high-speed grinding, the local area where the grinding wheel contacts the workpiece reaches temperatures above the austenitizing temperature. The application of cutting fluid during grinding is equivalent to another hardening process.
If the material contains alloying trace elements that increase the probability of crack quenching, the probability of crack grinding will increase.
Grinding cracks appear on the smooth, ground surface. Common types of crankpin cracks include:
Cracked cracks (shaped like a “Japanese mouth” or pit), as shown in Figure 7, which are single linear cracks, multiple parallel point-and-strip cracks, or a stack of point-and-strip cracks, as shown in Figure 8.
Single linear cracks or multiple parallel point cracks are distributed in the axial direction, perpendicular to the grinding direction. In the lateral projection, these cracks are generally radial, as shown in figure 9.
Figure 7 Cracks
Fig. 8 Multiple parallel points and strip cracks or a stack of point and strip cracks
Fig. 9 Lateral radial cracks
If a sample is taken perpendicular to this type of crack, the secondary temper structure can be observed under a metallographic microscope.
The metallographic characteristics of the secondary quenching layer from outside to inside are a bright white layer, a black-gray quenching layer (troostite) and an induction hardening layer (quenched martensite).
The hardness indentation size of the bright white layer can be used to determine its hardness, which is particularly high, as shown in Figures 10 and 11.
In some cases, only the tempered troostite layer can be seen and the bright white secondary temper layer cannot be seen.
The secondary tempering layer is very thin, requiring high sample preparation standards for metallographic samples. If sample preparation is not done correctly, the bright white layer may not be visible.
Fig. 10 Metallographic Structure of the Secondary Quenching Layer for Crack Grinding
Fig. 11 Secondary hardening layer hardness recoil comparison diagram for grinding cracks
(4) Appearance and identification of exposed non-metallic inclusions
Inclusions in steel are generally categorized into two types: metallic and non-metallic inclusions.
Metallic inclusions, which are typically external, can be avoided by implementing rigorous management practices and adhering to strict operating procedures.
On the other hand, non-metallic inclusions are formed by the reaction of gases in the steel, deoxidizers and alloying elements during casting, as well as the presence of refractory fragments.
To remove these inclusions during smelting, the liquid steel is fully boiled and stabilized in the steel ladle so that the inclusions can rise to the surface and be removed into the slag.
The position of non-metallic inclusions is not fixed and can occur singly or in clusters. As non-metallic materials are not magnetic, their presence disrupts the continuity of the material.
If the inclusions are exposed or are relatively close to the surface, they will appear as magnetic marks in magnetic particle flaw detection. The closer the inclusions are to the surface, the more prominent their magnetic trace will be. As a result, the magnetic traces of inclusions may be intermittent.
After forging, non-metallic inclusions are often distributed along the axial direction of the crankshaft. The lines of their magnetic strokes appear smooth and the end is often bald. When an inclusion is exposed after processing, it is considered an open defect (see Figure 12).
Fig. 12 Single open inclusion and enlarged morphology
A sample is taken perpendicular to the crack and examined under a metallographic microscope.
The depth of the fissure is not deep and its bottom has a round shape, as shown in Figure 13.
Fig. 13 Cross section without corrosion
4. Appearance and identification of magnetic traces of non-surface defects
After performing magnetic particle flaw detection, the magnetic suspension should be cleaned and observed with the naked eye under good lighting conditions.
If no defect is visible, the magnetic mark is considered a non-surface defect.
(1) Simplification of forging
During the forging process of forgings, the metal flows in a specific direction.
When dissecting the part, the forging flow lines can be observed through macro observation after corrosion, as shown in Figure 14.
Fig. 14 Macro image after alcohol etching with 5% nitric acid
During normal flaw detection procedures and as per specifications, a magnetic trace of the forging flow line is often not visible or very faint, as demonstrated in Figure 15.
Only when the magnetic field is very strong or there is segregation and a significant amount of inclusions present can a clear magnetic trace be seen.
Fig. 15 Front end of the crankshaft
(2) Segregation and inclusion
The non-uniformity of the chemical composition of different types of steel is called segregation.
Segregation can be divided into dendritic segregation, square segregation and point segregation based on the causes and manifestations.
Ingots, especially those made of medium-carbon chrome-molybdenum steel or chrome-nickel-molybdenum steel, often contain many inclusions, which are unavoidable.
The addition of alloying elements to steel typically reduces its fluidity, making it more difficult to remove non-metallic inclusions in alloy steel compared to carbon steel. This also increases the likelihood of segregation or inclusions.
During processing of an alloy steel crankshaft, metal flows from the center to the parting surface, resulting in segregation (strip) and inclusions that are generally most severe at the cut surface and closest to the surface.
Even if these inclusions are not exposed at the cutting edge, if they are close to the surface and are of a certain length, aerodynamic magnetic marks will appear during flaw detection, as demonstrated in Figure 16.
Figure 16 shows the current line in the section of a stereo crankshaft and segregation of strips on the surface after being macroetched with 5% nitric acid.
Alloy steel is known for its high hardenability, which makes it prone to producing structural heterogeneity (banded structure) during cooling.
Furthermore, alloy steel has a relatively low thermal conductivity, leading to increased residual stress in the steel.
If the grinding process is not carried out correctly, grinding cracks may occur in areas affected by these metallurgical defects.
5. Conclusion
Correctly identifying all types of magnetic marks in crankshaft fault detection requires extensive field experience for fault detection professionals and a unified understanding.
Currently, each engine factory has its own standard for detecting crankshaft faults.
There are varying understandings among professional manufacturers regarding the impact of magnetic marks from non-surface defects on the crankshaft fillet and journal and their effect on crankshaft fatigue performance. More research in this area is needed among industry peers.
