Effect of heat treatment on mechanical and magnetic properties of 0Cr13 steel

The effects of different recrystallization annealing and tempering processes on the mechanical and magnetic properties of 0Cr13 stainless steel at room temperature were studied. The results show that: 0Cr13 steel quenched at 980℃×1h, water-cooled +725℃×2h tempered, water-cooled +400℃×2h tempered, and furnace cooled, can obtain ferrite and martensite dual-phase organization, and the mechanical properties and magnetic properties match well; 0Cr13 steel quenched at 980℃×1h, water-cooled +725℃×2h tempered, water-cooled +870℃×2h tempered, furnace cooled, has excellent magnetic properties and less coercivity, but the strength is significantly reduced. The magnetic properties are excellent, and the coercivity is small, but the strength is reduced considerably. 820℃×5h furnace cooling recrystallization annealing can be obtained more regular, uniform equiaxial ferrite organization; the strength is lower than the tempering treatment but has good soft magnetic properties. The amount of ferrite in 0Cr13 steel is suitable for weak magnetic properties, and the amount of martensite is high for strength; there is a certain contradictory relationship between magnetic properties and strength, and better magnetic properties will lose some strength and vice versa.

0Cr13 steel is a low Cr content of stainless steel, has less variety of alloying elements, is low price, and usually has good corrosion resistance. Mechanical properties are widely used in shaft and rod-type devices, and corrosive environments mainly used to manufacture water vapor, ammonium bicarbonate mother liquor, hot sulfur-containing petroleum corrosion parts, equipment, etc. Duan Luzhao et al. studied the effects of different annealing processes on the mechanical properties of 0Cr13 steel. They found that the tensile strength showed an overall decreasing trend with increased annealing temperature and annealing time. The annealing temperature exceeded 810°C, the grain coarsening was serious, and the strength decreased significantly. Zhao Jiqing et al. studied the effect of FeCl3 solution on the pitting behavior of 0Cr13 steel and showed that the corrosion depth and quantity increased with increasing temperature, but compared with 304L austenitic stainless steel, 0Cr13 steel has better resistance to pitting. In addition, 0Cr13 steel can be used as a control rod drive mechanism; the control rod drive mechanism needs good wear resistance, toughness, and other mechanical properties but also has good magnetic properties; the magnetic lifting mechanism is the core drive mechanism of the control rod. 0Cr13 steel, as ferritic stainless steel, has excellent soft magnetic properties, but in the use of the process is difficult to meet the mechanical properties and magnetic requirements. However, it isn’t easy to meet the requirements of both mechanical and magnetic properties in the use process. Few articles study the effect of the heat treatment process on the magnetic properties of 0Cr13 steel. Therefore, this paper explores the changes in the organization, mechanical properties, and magnetic properties of 0Cr13 steel under different heat treatment processes, hoping to obtain a heat treatment process with the excellent matching of mechanical properties and magnetic properties.

1. Test material and method

The tested steel was smelted in a 50 kg vacuum medium frequency induction furnace with the chemical composition (mass fraction, %) of 0.024C, 13.20Cr, 0.13Ni, 0.59Mn, 0.23Si, 0.04P, 0.001S, and the balance of Fe, and cast into ingots. The effect of the tempering and recrystallization annealing heat treatment process on the mechanical properties and magnetic properties is studied, and the heat treatment regime is shown in Table 1. General requirements for the comprehensive mechanical properties of the parts usually need to be tempered to obtain a higher strength and sufficient toughness with the steel. The recrystallization annealing process is a heat treatment process to keep warm long enough to get close to the equilibrium organization. These two heat treatment processes can obtain better organization and performance.
Table.1 Heat treatment system

Sample status	Process No	Heat treatment process
Recrystallization annealing	1	820 ℃×3 h(AC)
	2	820 ℃×3 h(FC)
	3	820 ℃×5 h(AC)
	4	820 ℃×5 h(FC)
Quenching and tempering process	1	980 ℃ × 1 h quenching (WC)+725 ℃ × 2 h tempering (WC)
	2	980 ℃ × 1 h quenching (WC)+725 ℃ × 2 h tempering (WC)+870 ℃ × 2 h tempering (FC)
	3	980 ℃ × 1 h quenching (WC)+725 ℃ × 2 h tempering (WC)+400 ℃ × 2 h tempering (FC)

Note: AC – air cooling; FC – furnace cooling; WC – water cooling
The heat-treated bars are processed into standard mechanical and magnetic property test specimens for performance testing, followed by microstructure analysis. The mechanical property test includes the room temperature tensile test, impact test, hardness test, etc.; the magnetic property test mainly includes magnetic induction strength, coercive force, remanence, and magnetic permeability test under different magnetic field strengths, etc. The room temperature tensile test is performed by WE300B tensile tester, and the tensile specimen is a standard specimen of ϕ5mm×65mm, and the impact specimen is a standard U-notch specimen; the specimen for measuring magnetic properties is a circular specimen of ϕ40mm×ϕ32mm×10mm, as shown in Figure 1, and the magnetic induction strengths are measured at rated magnetic field strengths of 2500, 5000 and 10000A/m; microscopic The FEIQuanta650 scanning electron microscope was used for microstructure observation.

Figure.1 Schematic diagram of the circular specimen

2. Test results

2.1 Effect of recrystallization annealing process on mechanical properties and magnetic properties

The effects of recrystallization annealing time and annealing cooling method on the room temperature mechanical properties of 0Cr13 steel are shown in Fig. 2, and the dashed line in Fig. 2 is the expected value of the mechanical properties of the test steel. From Fig. 2, it can be seen that the plasticity of the test steel is higher under different recrystallization annealing processes, with a larger amount of richness, better toughness, and strength also meeting the requirements, but there are large differences. As shown in Figure 2 (a), using 820 ℃ × 3h air cooling recrystallization annealing, tensile strength and yield strength have a large amount of rich; extend the annealing time to 5h, tensile strength, and yield strength are reduced, the amount of rich are reduced; using furnace cooling annealing, strength than air cooling annealing have a certain degree of reduction. Recrystallization annealing aims to retransform the deformed grains into equiaxed grains while eliminating work hardening and residual internal stress. The steel organization and mechanical properties may return to their state before deformation, thus reducing the strength of the steel. The use of 820°C × 3h air-cooled recrystallization annealing also has a significant increase in impact absorption energy and hardness, but the material has low requirements for impact absorption energy and hardness, and all four annealing process-treated specimens meet the requirements for use.
The effect of the recrystallization annealing process on the magnetic properties of 0Cr13 steel at room temperature is shown in Fig. 3. The difference between the magnetic properties of 820℃×3h air-cooling and other recrystallization annealing processes is large, which may be related to its tissue state.
The effect of the recrystallization annealing process on the microstructure of 0Cr13 steel is shown in Fig. 4. From Fig. 4(a1,a2), it can be seen that the microstructure of 820℃×3h air-cooled annealing treatment is ribbon-like, with a large amount of fine martensite distributed at the grain boundaries, which is significantly different from the microstructure of 820℃×3h furnace cold annealing treatment, probably caused by the different original state before annealing. From Figure 4(b1,b2), it can be seen that the forging state organization after 820℃×3h furnace cold annealing treatment formed equiaxial ferrite organization, the original martensite organization has been obvious decomposition, internal precipitation of a large number of carbides, grain boundaries also precipitated a large number of coarse carbides; from Figure 4(c1,c2), it can be seen that the forging state organization after 820℃×5h air cooling annealing treatment, formed equiaxial Axial ferrite organization, grain boundary precipitation of a large number of carbide; Figure 4(d1,d2) shows that the microstructure of the forging organization after 820 ℃ × 5h furnace cold annealing treatment and 820 ℃ × 5h air cooling is the same, can not see obvious differences.

Figure.2 Effect of recrystallization annealing process on mechanical properties of 0Cr13 steel

Figure.3 Effect of recrystallization annealing process on the magnetic properties of 0Cr13 steel at room temperature
The results show that the 820℃×3h air cooling annealing treatment is ferrite and martensite organization, with more martensite organization, good mechanical properties, and richer, but the magnetic properties of martensite are worse than those of ferrite, so the magnetic induction strength after 820℃×3h air cooling annealing treatment is worse compared with the other three recrystallization annealing processes. The other three recrystallization annealing processes are all ferrite, and the magnetic properties are good when the ferrite content is higher. Although ferrite and martensite are ferromagnetic, ferrite is better than martensite in magnetic properties, mainly because the carbon content of ferrite is less, while the carbon content of martensite is more, is a supersaturated solid solution, in the transformation of austenite into martensite, the internal carbon can not be precipitated. The solid solution in the matrix, so that the lattice distortion, in the magnetization of the supersaturated structure of martensite on the movement of the magnetic domains, plays a hindering role, the magnetic fields Not easy to move, the magnetization is difficult, so the magnetic properties are weaker than ferrite. In addition, the homogeneity of the tissue and the size are also important factors affecting the magnetic properties. As shown in Fig. 4(d1,d2), the tissue after cold annealing at 820℃×5h furnace is equiaxed ferrite, which is more regular, homogeneous, and moderate in size, so the magnetic permeability is high, and the magnetic induction intensity is also high.
It can be seen that recrystallization annealing can be obtained equiaxial crystal organization, magnetic properties are better, especially 820 ℃ × 5h furnace cooling, the highest magnetic induction strength, and coercivity is small, remanent magnetism is large, the comprehensive magnetic properties are increased. However, the mechanical properties will be reduced, although to meet the requirements, the richness is not large. Therefore, the strength can be further improved by optimizing the annealing process.

Figure.4 Effect of recrystallization annealing process on the microstructure of 0Cr13 steel

Figure.5 Effect of the quenching process on mechanical properties of 0Cr13 steel

2.2 Influence of tempering process on mechanical properties and magnetic properties

The effect of the tempering process 980℃×1h water cooling +725℃×2h water cooling, and the effect of the different tempering processes on the mechanical properties of 0Cr13 steel at room temperature is shown in Fig.5. It can be seen from Fig.5 that the strength and plasticity of 0Cr13 steel meet the requirements of the use conditions under different tempering processes, with better plasticity and higher toughness. The difference in hardness is not significant, but the difference in strength is obvious.
As can be seen from Figure 5(a), the strength has a larger affluence after treatment with the standard tempering process; after tempering treatment and then tempering treatment at 870℃×2h furnace cooling, the strength decreases significantly, while after tempering treatment with tempering + 400℃×2h furnace cooling, the strength value is slightly higher than after tempering treatment. 0Cr13 steel grade belongs to low carbon steel, tempering treatment with high-temperature tempering, although the elimination of The 0Cr13 steel grade is low carbon steel. The workpiece’s quenching stress is eliminated by tempering at a high temperature after tempering. However, the internal stress is eliminated, the strength is greatly reduced due to the rapid growth of grains, and the test steel is tempered at low temperatures after tempering. The workpiece’s quenching stress is eliminated, and the 0Cr13 steel maintains a good elastic limit, high strength and hardness, and good plastic toughness.

Figure.6 Effect of the tempering process on the magnetic properties of 0Cr13 steel at room temperature

The effect of the standard tempering process (980℃×1h water cooling + 725℃×2h water cooling) and different tempering processes after tempering on the magnetic properties are shown in Figure 6. After tempering at high temperatures, the magnetic induction strength is increased more. When the magnetic field strength exceeds 5000A/m, the magnetic properties are better, and the magnetic induction strength values do not differ much. Generally, the external magnetic field generates the magnetism of ferromagnetic materials. The force of the external magnetic field affects the ferromagnetic materials, and the magnetization phenomenon occurs, thus showing the magnetic properties. Magnetization is the phenomenon of the movement of the magnetic wall and the regular arrangement of the magnetic moments of the magnetic domains under the action of the external magnetic field force, but because the structure of the material will have an obstructing effect on the movement of the magnetic wall, making it difficult to move the magnetic wall, thus showing the difference in magnetic properties. Therefore, the strength of magnetism depends on the structure of the material itself and the strength of the external magnetic field. When the structure is fixed, the material magnetism increases with the external magnetic field strength increase. From Fig. 6(b), it can be seen that the remanent magnetism of the three tempering processes is low, and the difference is not large, among which the remanent magnetism of tempering + 870°C high-temperature tempering is the lowest; the coercivity difference is large, and the coercivity of tempering + 400°C low-temperature tempering is higher, while the coercivity of tempering + 870°C high-temperature tempering is significantly lower.

Figure.7 Effect of the tempering process on the microstructure of 0Cr13 steel
The microstructure of 0Cr13 steel after different tempering process is shown in Fig. 7. As can be seen from Fig. 7, after the standard tempering process, the microstructure of 0Cr13 steel is ferrite and tempered martensite two-phase organization, with a large number of fine grains distributed at the grain boundaries; after tempering +870℃×2h furnace cold tempering treatment, a large number of fine martensite organization appears at the grain boundaries, and after magnification, it can be seen that the martensite organization is not obvious The slate-like, but the precipitation of a large number of coarse carbide, may be in the 870 ℃ tempering, on the one hand, the precipitation of coarse carbide, on the other hand, in the grain boundary of α → γ transformation, the formation of fine austenite organization, the cooling process formed martensite organization; tempering + 400 ℃ low temperature tempering treatment, the microstructure of a large number of fine martensite observed, and more uniform distribution.
After tempering +870℃ high-temperature tempering, coarse carbides precipitated at the grain boundaries, resulting in a much lower strength than the tempered organization. After tempering, the microstructure is ferrite + a large amount of martensite, martensite ferromagnetism is weak, so the magnetic properties in this state are poor. In addition, the grain size, grain state, and internal stress of the material will also affect the magnetic wall movement and the arrangement of the magnetic moment of the magnetic domain; small grains, not eliminating internal stress, will increase the resistance to magnetization, resulting in the difficulty of magnetization, especially when the applied external magnetic field force is small. Tempering + 400°C low-temperature tempering, eliminating some of the internal stress, makes the magnetic properties improve; tempering + 870°C high-temperature tempering, the ferrite grains grow, and the obstruction to the magnetic wall and magnetic moment is reduced, making the magnetic domains move easily, compared with tempering + 400°C low-temperature tempering, it is easier to magnetize, and the magnetic induction strength is higher under the weak magnetic field of 2500A/m.

3. Discussion

Both ferritic and martensite in stainless steel are ferromagnetic substances with better magnetic properties, and the material is easily magnetized. Since martensite is a supersaturated tissue, the hindrance to the magnetic moment is greater, so the magnetic properties of martensite are weaker compared to ferrite. By comparing different heat treatment processes, the tissue obtained after recrystallization annealing is a uniform equiaxed grain with the highest magnetic energy after furnace cooling at 820°C×5h; the tissue after tempering treatment contains some martensite, the magnetic energy is weakened, and the magnetic induction intensity is smaller under weak magnetic field. In terms of mechanical properties, martensite’s strength is higher than ferrite’s. From the test results (Figure 4 and Figure 7), it can be seen that the microstructure after recrystallization annealing treatment is mostly single-phase ferrite with less hardness. In comparison, the organization after tempering treatment is all ferrite + martensite two-phase organization with higher hardness.
0Cr13 steel as structural parts needs to meet specific strength requirements and as a soft magnetic material, but also to be easy to magnetize, with good magnetic properties. Usually, the strengthening of metal materials can be achieved in two ways: (i) to obtain complete grains without defects so that the actual strength of the metal is close to the theoretical value; (ii) to increase the dislocation density in the defective grains to achieve the strengthening effect. However, obtaining intact grains is very difficult, so increasing dislocation density, refining grains, and generating second-phase particles are commonly used strengthening methods. Still, the use of these methods is detrimental to the magnetic properties. Magnetism is generated mainly through the action of an external magnetic field, which exerts a certain force on the inner magnetic wall of the material to displace the wall, thus causing the material to become magnetic. Internal stresses in soft magnetic materials, second-phase particles, and non-uniform areas of the material can impede the movement of the domain walls and weaken the magnetism. In summary, steel’s magnetic properties and mechanical properties are a mutual check and balance relationship, and reasonable control of the organization is also required to obtain a material with well-matched magnetic properties and mechanical properties.
As shown in Fig. 7, the tempering + 400℃ low-temperature tempering, the ferrite has less tendency to grow and precipitates a large amount of fine martensite tissue, which increases the strength; in addition, the diffuse small martensite particles also play the role of second phase strengthening, which further enhances the material strength, making it stronger than the standard tempered specimens. Tempered + 870℃ high temperature tempered specimens. In addition, the organization becomes homogeneous after low temperature tempering at 400°C for the magnetic properties. The internal stresses are eliminated, and the lattice distortion and defects in the martensite are reduced, which makes the magnetic wall movement less obstructed, and the magnetic wall movement more flexible under the action of the external magnetic field. The soft magnetic material is easier to magnetize, and the magnetic properties are improved.
In addition, it can be seen from Figure 6 that the magnetic induction strength of this soft magnetic material is significantly increased after tempering in a weak magnetic field of 2500 A/m. Both the standard tempering treatment followed by tempering treatment can reduce the stress and lattice distortion of martensite, so tempering treatment can significantly increase magnetic induction strength at a weak magnetic field, but tempering at 870°C will cause a significant decrease in strength. In addition, tempering temperature also affects the size of ferrite tissue, which also affects the strength value. 870℃ high-temperature tempering, the ferrite tissue grows rapidly, the grain size is coarse, and the strength value decreases, while 400℃ low-temperature tempering, the ferrite tissue has less tendency to grow, so the strength of 400℃ low-temperature tempering is higher when comparing the two. Comprehensive consideration of magnetic properties and strength, by the standard tempering treatment and then 400 ℃ low-temperature tempering, can be obtained a better overall performance of the organization.
The test of several heat treatment processes, 820 ℃ × 5h furnace cold recrystallization annealing process is a relatively simple and economical heat treatment process; although the strength is lower than the tempering treatment, the organization is mostly uniform equiaxed grains, in the weak magnetic field magnetization is easy, excellent magnetic properties can be used as a strong magnetic component with low requirements for mechanical properties. And the standard tempering treatment followed by a 400 ℃ low-temperature tempering process, the comprehensive performance is better, the strength and magnetic properties are richer, and in the weak magnetic field of 2500A/m, the magnetic induction strength can also reach a higher level.

4. Conclusion

The following conclusions were obtained from the study and analysis of the effect of heat treatment on the mechanical properties and magnetic properties of 0Cr13 steel.

• 1) Standard tempering treatment (980℃×1h quenching, WC+725℃×2h tempering, WC) followed by low temperature tempering at 400℃ can obtain ferrite and martensite two-phase organization and obtain a better match of comprehensive strength and magnetic properties; standard tempering treatment followed by high temperature tempering at 870℃ will improve the magnetic induction strength, but the strength will decrease significantly.
• 2) Uniform equiaxial ferrite organization can be obtained after recrystallization annealing treatment with 820℃×5h furnace cooling. The strength is lower than that after tempering treatment, but it has good magnetic properties.
• 3) High temperature tempering at 870℃ after tempering treatment can significantly reduce the coercivity Hc and residual magnetic induction Br of the material, which can be quickly demagnetized when de-magnetizing.

Authors: Yuanyuan Ma, Hansheng Bao, Zhihua Gong, Jiqing Zhao, Gang Yang

Source: China 15CrMo Forgings Manufacturer – Yaang Pipe Industry (www.epowermetals.com)