Metallographic Inspection Knowledge of Bolt Quenching And Tempering

---

1. Quenched and tempered metallographic examination
Quenching and tempering is to heat the hypoeutectoid steel above Ac3 by 30-50°C. When the vacuum quenching temperature of the workpiece is normal, the holding time is sufficient, and the cooling rate is also high, and the supercooled austenite does not decompose during the vacuum quenching process, then The microstructure obtained after vacuum quenching should be lath martensite and needle lamellar martensite. During the medium-high temperature tempering process, carbides are precipitated in the martensite to obtain tempered sorbite or tempered troostite.
1.1 Quenching and tempering organization and evaluation
In order to ensure that high-strength bolts are fully austenitized during vacuum quenching, the vacuum quenched structure is uniform, and there is no undissolved ferrite and non-martensitic structure, full attention must be paid to the metallographic examination of the vacuum quenched structure. For high-strength bolts of grade 10.9 and above, the uniformity of the vacuum quenching structure is particularly important.
Vacuum heat treatment of high-strength bolts attaches great importance to the full austenitization of steel to ensure the uniformity of its structure, so as to obtain the best combination of strength and toughness, and ensure the safety and reliability of bolts in service. Manufacturers of high-strength bolts have not yet paid enough attention to this. The common problems are insufficient vacuum quenching, heating and heat preservation of bolts, and insufficient austenitization.
The standard requires that the sample should be obtained mechanically in a cold state; if hot cutting is used, the heat-affected zone must be completely removed. In the process of sample preparation, there should be no tissue changes caused by heat. After the sample is polished, it is etched with a 2% to 5% nitric acid alcohol solution by volume.
The metallographic structure is evaluated according to grades 1 to 8, grade 1 is the best and grade 8 is the worst. The third group of rating charts of this standard is suitable for quenched and tempered parts of structural steel, especially the quenched and tempered inspection of high-strength bolts.
When the evaluated quenched and tempered tissue is between two levels, the next level shall be the judgment level, for example, if it is greater than level 3 but less than level 4, then it shall be judged as level 4. The quenched and tempered metallographic structure analysis is observed under an optical microscope at 500 times, and the qualified level is negotiated and agreed by the supplier and the buyer. If there is no agreement, the qualified level is 1 to 4. Production practice shows that for bolts serving in low temperature environments, grades 1 to 3.5 are the acceptance criteria.
In order to ensure good hardenability, for bolts with grade 8.8 and thread diameter exceeding 20mm, alloy steel materials specified in the standard must be vacuum quenched and tempered.
Due to the different austenitizing temperature, the shape and size of martensite are different. Grade 1 belongs to the low austenitization temperature, and the vacuum quenching structure is cryptoneedle martensite, fine needle martensite and ferrite with a volume fraction not greater than 5%; while grade 8 belongs to the superheated structure, which is a coarse plate Strip martensite + coarse needle martensite.
During normal vacuum quenching, it is controlled at grades 3 to 5, and its structure is fine lath martensite + sheet martensite. Grade 6 has higher impact toughness, yield strength and tensile strength, and is suitable for larger specifications and Bolts requiring high hardenability.
Figure 1 shows the 4th grade of strip and acicular martensite in 40Cr steel plate, Figure 2 shows the 5th grade of 45 steel plate strip and acicular martensite, and Figure 3 shows the 4th grade of SCM435 steel plate strip and acicular martensite, all of the above For normal vacuum quenching weaving.

Fig.1 Lath and acicular martensite (40Cr, grade 4)

Fig.2 Lath martensite mixture

The tempered sorbite structure is actually extremely small granular carbides distributed on the α-phase matrix, as shown in Figure 4 and Figure 5. The tempering temperature is generally 450-600°C according to the grade requirements of high-strength bolts, and the specific temperature range varies depending on the chemical composition of the steel.
Because the addition of alloying elements will slow down the decomposition of martensite, the precipitation and aggregation of carbides, and the transformation of retained austenite, the tempering temperature will move to a higher level.

Fig.3 lath and acicular martensite         Fig.4 tempered sorbite

2. Quenched and Tempered Defective Tissue
2.1 Vacuum quenching superheated structure
The high heating temperature of vacuum quenching causes the growth of austenite grains, and a coarse martensite structure is obtained after vacuum quenching, as shown in Figure 6. Once coarse acicular martensite appears, good comprehensive mechanical properties cannot be obtained even if a reasonable vacuum tempering temperature is used for tempering, and bolts with this structure are very likely to fail early due to fracture during use.

Figure 5 Tempered sorbite         Figure 6 Coarse martensite structure (42CrMo, grade 8)

The normal vacuum quenching temperature of SWRCH35K does not exceed 870°C. When the sample in Figure 7 is vacuum quenched at 920 °C, the grains grow rapidly, and the parallel martensite orientations in different grains are different, and the austenitization increases accordingly, and the residual austenite is relatively more after vacuum quenching. Coarse martensite has poor comprehensive performance and is easy to quench and crack.

Fig.7 Coarse martensite and a small amount of retained austenite

Fig.8 Martensite and undissolved ferrite

2.2 Vacuum quenching underheated structure
The structure obtained after normal vacuum quenching of high-strength bolts should be lath martensite and needle-like martensite. Underheating of vacuum quenching is that the heating temperature of vacuum quenching is too low or the heat preservation is insufficient, and the austenite is not homogenized, resulting in the structure after vacuum quenching of martensite and undissolved ferrite, as shown in Figure 8. Ferrite cannot be eliminated even by tempering.
45 steel is kept warm in Ac1～Ac3, and the ferrite in Figure 9 remains in the matrix. At the same time, due to the low vacuum quenching temperature, short holding time, and poor homogenization of austenite, troostite transformation occurs locally during cooling.
2.3 Underhardened structure
The vacuum quenching temperature is normal and the holding time is sufficient, but the cooling rate is not enough to harden through. As a result, different structures will be obtained along the cross-section of the workpiece. Even if the surface is martensite, non-martensite structures will gradually appear in the center. Non-martensitic organizations include troostite, bainite, etc., and the core is troostite and ferrite.
The non-martensitic structure that appears in low alloy steel is generally not troostite, but upper bainite, as shown in Figure 10. The organization is a small amount of bainite distributed on the martensite matrix, and it is easier to detect this defect by metallographic methods.

Figure 9 Light gray martensite + black mass flocculent troostite + white block granular ferrite
Figure 10 Tempered sorbite + upper bainite in the core structure

The shape of medium carbon quenched and tempered structure depends on the vacuum quenched state structure. The massive ferrite remaining in the martensite structure due to insufficient heating, or the precipitation of network or semi-reticular ferrite at the grain boundary due to insufficient cooling, are all is harmful.
It is worth noting that there is a difference in morphology between the undissolved ferrite caused by insufficient heating and the first precipitated ferrite caused by insufficient cooling. The latter is newly formed proeutectoid ferrite, which is relatively fine in shape and distributed on the austenite grain boundary (Fig. 12).

Fig.11 Vacuum quenching underheated structure (40Cr)

Fig.12 Vacuum quenching insufficient cooling structure (40Cr)

Vacuum quenching defects are the most common in bolt quenching and tempering vacuum heat treatment process, such as insufficient hardness, deformation, cracking and so on. There are many reasons for defects, which need to be analyzed from various aspects. Metallographic inspection is a commonly used method.
3. Common vacuum quenching defects in vacuum heat treatment
3.1 Vacuum quenching cracks
The internal stress caused in the bolt during vacuum quenching is the root cause of deformation and cracking. When the internal stress exceeds the yield strength of the material, it causes deformation; when the internal stress exceeds the fracture strength of the material, it causes cracking. Only tensile stress is a necessary condition for crack initiation and propagation. The reasons for vacuum quenching cracks can be considered from two aspects, one is what factors cause greater stress; the other is whether there are defects in the material, resulting in a decrease in strength and toughness.
3.1.1 Characteristics of vacuum quenching cracks
In most cases, the cracks propagate from the surface to the center, and the macroscopic shape is relatively straight.
There is no decarburization on both sides of the crack from the macroscopic and microscopic views, but if the high temperature tempering is carried out in an oxidizing atmosphere, there will be an oxide layer on both sides of the vacuum quenching crack (Figure 13).

Fig.13 Severe decarburization on both sides of the crack

Fig.14 Surface decarburized ferrite and low carbon martensite

3.1.2 Increased internal stress causes vacuum quenching cracks in normal tissues
If the design is unreasonable, if there are sharp corners and sudden changes in cross-section, it is easy to cause stress concentration.
The cooling is too strong. For example, the rapid vacuum quenching oil should be used, but the aqueous solution is chosen, and it should be cooled thoroughly when it should not be cooled.
Improper cooling method during vacuum quenching, vacuum tempering not carried out in time after vacuum quenching
3.1.3 Vacuum quenching cracks caused by structural defects
The vacuum quenching temperature is too high, the austenite grains are coarse, and the coarser martensite is formed after vacuum quenching, which is easy to crack. Especially the coarse high-carbon martensite is often accompanied by microcracks.
There are brittle phases such as network carbides in steel, and it is easy to crack along the brittle carbide network during vacuum quenching. There is a carbide network distributed along the grain boundary at the grain boundary, and it is also easy to wear and tear during grinding.
Steel has defects such as folds or coarse inclusions, and cracks are easy to form along these defects during vacuum quenching.
There is serious segregation in the steel, the structure is uneven after vacuum quenching, the internal stress is large and uneven, and it is easy to crack.
Due to the decarburization of the surface of the fastener, the volume expansion of the surface layer is small during vacuum quenching, and it is easy to form cracks under the stress of two phases.
3.2 Insufficient hardness of vacuum quenching
If the heating temperature is insufficient, troostite will be formed during cooling. When there is little troostite, there will be no obvious change in hardness, but the metallographic structure is easy to identify.
The cooling rate of vacuum quenching is insufficient, and in addition to martensite, there are troostite or bainite in the vacuum quenching structure. The more troostite or bainite, the lower the hardness.
The surface is decarburized, and it is not easy to form martensite or low-carbon martensite during vacuum quenching, as shown in Figure 14.
Vacuum quenching is overheated, the overheated structure has coarse martensite, the amount of retained austenite increases significantly, and the hardness also decreases.
The performance of bolts after vacuum quenching and tempering cannot meet the technical requirements. There are many influencing factors, and its quality analysis is a complicated process. Here it is only emphasized that metallographic inspection should pay attention to whether there are unfavorable factors affecting strength and toughness, such as coarse grains, non-metallic inclusions, reticular cementite, reticular ferrite, and non-martensitic structures in vacuum quenching structures. And micro-segregation in low-magnification detection caused by uneven organization, etc.

Learn More:

Ion Nitriding vs Gas Nitriding

Vacuum Heat Treatment Of Fasteners

Sintering of B4C by Pressureless Liquid Phase Sintering