42CrMo is a high-strength alloy steel with high strength and toughness, good comprehensive mechanical properties, high hardenability, and no obvious temper brittleness.
After quenching and tempering, it has high fatigue limit, impact resistance, and good low-temperature impact toughness. It is commonly used to manufacture large and medium-sized steel pieces that require a certain strength and toughness.
Our company selects 42CrMo steel for manufacturing a large pivot, and the process route is: blank forging → normalizing → rough machining → quenching and tempering → finishing → induction hardening excircle → grinding excircle.
The pivot structure is shown in Figure 1.
Fig. 1 Pivot Structure
2. Overview of defective parts
Cracks occur on the pivot shaft during quenching, tempering and quenching.
The crack is located in the transition arc area between the pivot shaft root and the flange.
The whole pivot shaft breaks along the radial direction and extends to the big end flange.
The whole pivot shaft cracks. The crack shape and location are shown in Fig. 2.
Fig. 2 Crack morphology after quenching, tempering and quenching
The quenching and tempering process of this piece is: 840 ℃ holding for 3h, oil cooling quenching after being taken out of the furnace, quenching time is 30min, and Houghton K oil is used as the quenching oil.
Many batches of this type of product have been produced without cracks.
In order to analyze the causes of cracks, the cracked workpiece was sampled at the big end flange, and the chemical composition, metallographic structure and cracks were analyzed.
Take samples from the flange crack (see Fig. 3) for analysis.
Fig. 3 Sampling at flange
See Table 1 for chemical composition analysis results
Table 1 Chemical Composition (Mass Fraction) of 42CrMo Steel Pivot (%)
|Element||Standard value||Measured value|
The metallographic structure is shown in Fig. 4.
Fig. 4 Metallographic Structure of Crack and Surface
The analysis results show that the chemical composition meets the technical requirements of 42CrMo steel;
The measured surface hardness is 296HBW (meeting the required value of 277-331HBW);
The metallographic structure is tempered sorbite, and the central structure is tempered sorbite+ferrite.
The crack of the workpiece is arc-shaped, and there is no decarburization on both sides of the crack, which proves that there is no crack before quenching and tempering, and no other detailed abnormality is found.
The physical and chemical analysts judged that the crack was caused by excessive stress during quenching, tempering and quenching according to the sample.
In view of this problem, the author believes that, due to the large size of this piece and the difficulty in sampling, the sample cutting position is selected as the easy sampling area, which may be the crack growth area rather than the crack initiation location, and further analysis should be carried out.
Because the pivot shaft is not completely broken, the crack source cannot be found through the crack fracture surface.
In order to further analyze the cause of the crack, it is proposed to cut the workpiece to find the crack source. Flame or plasma cutting may have an impact on the fracture location.
Therefore, wire cutting is adopted to cut the crack location at the shaft transition zone of the faulty part.
When the cutting reaches 1/3 of the diameter, the pivot shaft breaks into two sections.
3. Analysis of crack causes
The fracture morphology after wire cutting is shown in Fig. 5.
Fig. 5 Crack Fracture Position after Wire Cutting
The heart of the fracture is brown, and the outside is normal metal color.
The brown color of the heart is abnormal. It is inferred that the brown color may be caused by oxidation.
The analysis shows that there should have been cracks in the center of the part before quenching, tempering and quenching, and after the crack is formed, it will be oxidized during subsequent forging, normalizing and quenching, so it will appear abnormal brown.
To verify this judgment, the brown area was sampled and analyzed.
The sample is cut from the center along the axial direction.
After cutting and processing, it is found that there are many cracks distributed along the radial direction, as shown in Fig. 6.
Fig. 6 Crack Fracture Location
The analysis of the sample shows that the sample has an obvious banded structure (Grade 4), as shown in Fig. 7.
Fig. 7 Metallographic Structure of the Crack Center
The cracks are distributed along the banded structure direction, multiple and roughly parallel, and perpendicular to the forging direction.
The author thinks that there are cracks before the quenching and tempering cracking of the pivot shaft, and the cracks exist in the center, and there are no cracks on the surface.
During the quenching and tempering, the cracks expand due to the influence of organizational stress and thermal stress, and the banded structure also has some influence.
Therefore, it is necessary to further analyze whether there are problems such as insufficient forging temperature and unreasonable forging ratio in the forging process.
In order to prevent the problem parts from entering the market, the in-process products were investigated.
All the in-process products were NDT tested with ultrasonic flaw detector.
Two abnormal products were found, and the production serial number was checked.
The production serial number of the two abnormal products and the cracked parts was the same batch of forged products.
In order to verify the early analysis and judgment, wire cutting was carried out along the axial direction for the abnormal parts found.
It can be clearly seen from the cutting surface that there are several detailed cracks inside, which are perpendicular to the forging direction, as shown in Fig. 8.
Fig. 8 Cutting surface of defective parts
Through further verification, it was judged that the crack was generated during forging.
1) The direct cause of the fracture of the pivot shaft should be the forging crack formed during the forging process of the piece.
The crack expanded during the quenching, tempering and quenching, and the existence of banded structure also has some influence on the crack.
2) During fault analysis, the crack source should be carefully found.
Different sampling locations have a great impact on the analysis results.
Therefore, the cause should be carefully found and analyzed from the source to avoid the wrong direction.
3) The incoming inspection of raw materials shall be strengthened to prevent defective parts from entering the production process