2016 International Conference on Advanced Materials Science and Technology (AMST 2016)

ISBN: 978-1-60595-397-7

Tempering Quenching on Low-Alloy Structural Steel at Different Temperature Changes and Performance Analysis

Kang-cheng DONG1, Ming-zhu LI1, Li-Juan ZHANG1 and Feng DONG2

1Shaanxi Xijing College 710123 Xian

2Xian Institute of Thermal Engineering, Ltd

Keywords: 15CDV6 steel, Quenching and tempering, Organization, Analysis.

Abstract. 15CDV6 aviation and space industry development of high toughness and high-strength low-alloy structural steel. Aviation, space structures under different temperature tempering, quenching oil is typically used to meet different performance requirements. But there is no systematic study of the different oil quenching and tempering of steel microstructure and mechanical properties change, the purpose of quenching and tempering effect of microstructure on mechanical properties and analysis of organizational change, analysis of the different temperature of the steel, steels and heat treatment process of evaluation studies.

Test Method

Test with 5 mm thick hot-rolled plate, its chemical composition see Table 1.

Table 1. 15CDV6 steel chemical composition.

Chemical elements C Cr Mn V Mo Si P S

The content of（Wt%） 0.157 1.36 0.89 0.36 1.03 0.13 0.012 0.0088

All the specimens were cut along the rolling direction. Heat temperature specification is:

970~980 ℃ of fully austenitic, oil quenching after heat for 40 minutes, respectively, in 200~700℃ differing temperature tempering after 1 hour, after tempering air cooling.

High energy impact specimen using v-notched specimens at different temperature tempering toughness. In 1195-determination of tensile properties on a test machine. Scanning electron microscopic observation on PSEM-500 morphology of impact specimen notch at a time. Observation by transmission electron microscopy and electron diffraction using metal film, extraction replica and two-level complex type samples, transmission electron microscopy, JEM-200 and DXA4-10.With a magnetic field strength of 3000 OST magnetic Tester tempering residual austenite at different temperatures.

Results and Discussion Microstructure Analysis

Figure 1. After quenching oil thin film diffraction contrast organization 22000 x.

87

15CDV6 steel quenched lath marten site is the main body, and there is a certain amount of Bain ite, shown in Photo 1. Lath marten site has since tempered. Ferro-meter oil quenching about 3.6% of residual austenite in the Organization, and film distribution section of lath marten site or between island organizations.

With the tempering temperature of residual austenite quantity changes as shown in Figure 1, known from the diagram, 300℃, has broken down.

Figure 2. A total of 1 15CDV6 not temperature after quenching oil Tempering residual austenite quantity changes.

Photo 2-6 for different tempering temperature of electron microstructure. At 200 ℃ tempered marten site has a large number of dispersed fine carbides (Photo 2).When you 300 ℃, lath marten site carbide precipitation in the body shape of a fine needle, showing widman stat ten-dispersed, using electron diffraction analysis showed that this type of widman stat ten carbide cement ite. More than 300 ℃ tempered is a striking feature of the Microstructure distribution flaky precipitation along the border strip which appeared in confirmed by electron diffraction and dark field analysis of cementite. Based on the distribution of retained austenite, the slats cement ite is mainly formed by decomposition of residual austenite. Photos from 5a to be seen, changes of Bainite during tempering is very small. To note here is that regardless of lath martensite in the precipitation of fine needle cement ite and lamellar cement ite is via 625 ¡æ of tempering is not obvious when the ball. When tempering temperature 700 ℃, the cementite to gather into a ball and obviously grew up (Photo 6), ferrite replies, but not yet evident recrystallization.

Figure 3. 200 ℃ tempered two level complex type organization of 1000 x.

88

A. thin film Diffraction contrast organization 22000X B. extraction replica

Figure 4. 300 ℃ electron metallographic organization.

A. Microstructure of a bright-field B. Cementite field organization

Figure 5. Of 450 ℃ thin film Diffraction contrast organization 28000 x.

A. thin film Diffraction contrast organization 26000 x B. extraction replica 10000 x

Figure 6. 625℃ tempering microstructure of electronic.

89

Figure 7. 22000 photos 6 700 c thin film Diffract ion contrast organization x.

Containing molybdenum and vanadium steels temperature tempering Mo2C, VC and other carbides precipitation caused by secondary hardening, first before the release of these compounds are the processes of dissolution of cementite, and then control the formation of these compounds. Tests are not found in the carbides Mo2C, VC and other precipitation, is probably due to the effects of alloying elements makes cement ite stable, in the case of tempering time shorter is not dissolved. This special alloy carbides small amount, the size is very small, resulting in electron diffraction methods it is difficult to tell.

Strength and Hardness Changes

Tempering temperature and mechanical properties of relationships as shown in Figure 2. Knowledge, strength and hardness changes can be seen [1], 15CDV6 steel quenched tissue has a high temper resistance, but with no obvious secondary hardening phenomenon. Secondary hardening of steel is due to the dispersed Mo2C, VC and other special carbide alloy instead of cementite results. Organizational analysis in front of knowledge, can be considered due to the higher temperature (~625 c) tempering alloy cementite precipitation is still fine-needle-shaped, without dissolving, which makes steel showed higher tempering resistance; And did not release a large number of special alloys with high dispersion of compounds, which led to the obvious secondary hardening effect. Temperatures up to 700 °C tempering strength and hardness greatly decreased significantly softened, ductility and toughness increased significantly [2].This further proves that the mechanical properties changes coincide with the cementite in ball and accumulation of growing up.

In Tempered Mar Tensite Embrittlement

In Figure 2 is not difficult to see that the impact toughness at less than 300℃, Tempering at basically no change, when more than 300 ℃Began to decline when, tempered marten site embrittlement occurs (the first class Temper brittleness). Tempering temperature 650 ℃ after the fire, obvious improvement in toughness, proved more High temperature tempering toughness reduced same is not reversible and does not belong to the second category temper brittleness. A rush SEM observation of fracture-click test show that oil Low temperature fracture toughness of tempered and 200~250 ℃ Fossa (Picture 7A), tempered mart ensite embrittlement temperature range Dominated by dimples and cleavage patterns (Photo 7b). From Fracture analysis of gathered material is porous-ductility fracture Cracking and toughness reduced accordingly less brittle is not serious. If by Impurities caused by the segregation of tempered marten site mbrittlement and fracture characteristics along the c-type. Within the scope of the experiments in tempering temperature Characteristics of inter granular fracture was found. Therefore, 15CDV6 steel back Marten site embrittlement of fire was not caused by impurity segregation.

90

Figure 8. Different temperature mechanical properties of steel oil quenching and tempering Changes (975 ℃ )

austenitizing kept for 40 minutes oil co cooling; Tempering one hour. Figure 2 15CDV6.

A. 200 ¡æ tempering 640 x B. 625 ℃ 320 x

Figure 9. Photos 7 of SEM fracture morphology.

Figure 2 Figure 1 and compare found clear of residual austenite decomposition and temperature of tempered mar tensite embrittlement appear largely corresponds to the range. Decomposition of residual austenite between the slats of the formation of cementite, impact loads, will become the core of the cracks are easy to form and easy to extend the convenience channel, with the result that toughness decreases [3]. Tempered martensite embrittlement along the stretch until 625, apparently between the slats and cementite at this temperature tempering after the sheet is about. Therefore, 15CDV6 steel tempered marten site embrittlement can be considered mainly due to the tempering of retained austenite decomposition between the planks that formed due to cementite, which is consistent with the results of other steel.

Study on granular Bainite in 15CDV6 steel found, tempering embrittlement of granular Bainite is mainly caused due to the decomposition of the residual austenite and temper embritt lement of granular Bainite to be much more serious than the brittle martensite (а k reduction measure, the former is 9-10Kg-m/cm2, which is only ~2Kg-m/cm2).Corresponding granular Bainite with the amount of residual austenite in martensite in the Organization (which is about 10.5%, which is 3.5%).Although the two organizations are not the same, but brittle is mainly attributable to a decomposition of the residual austenite. Of course, size, there are other factors affecting the brittle, 15CDV6 steel of different types of organization studies, brittle tendencies of retained austenite in size and the amount of, and closely related to the degree of carbon-rich. In quenching microstructure, residual austenite more brittle and more serious. Thus improving the stability of retained austenite will effectively tempered martensite embrittlement temperature range to higher temperatures, which can be achieved by increasing the content of Silicon in steel.

91

Results of this experiment showed that 15CDV6 steel after quenching oil at 200-250 cold tempered high strength and impact toughness, particularly those with high strain low cycle impact fatigue life and fatigue life, as shown in table 2. High strength material, high toughness and the fatigue life is of important practical significance. Therefore, the structural part is used at room temperature and low temperature tempering process to research and business.

Table 2. 15CDV6 steel after quenching oil tempering of fatigue life.

Tempering temperature(℃) Impact fatigue life(KG/CM2) Strain low-cycle fatigue life(Hour)

200 18500 2380 625 13810 1060

Conclusions

1) 15CDV6 steel quenching oil mainly to lath martensite and Bainite under a certain amount. Mainly of residual austenite from lath martensite, more than 300 ℃ tempered when fundamental decomposition, between the lath marten site formation of cementite.

2) Tempered martensite embrittlement is mainly due to the residual austenite between the slats in tempering decomposition caused by the formation of cementite.

3) 15CDV6 steel after quenching oil at 200-250 cold tempered with good mechanical properties. Used for room temperature structure could consider low temperature tempering process.

References

[1] Long Qing improved 15CDV6 2001 discussion on the technological properties of low-alloy steel 9-14 1th.

[2] Zhang Juzhou heat treatment process on mechanical properties and microstructure of Nb-Ti low alloy steel casting technology of 2014, 1th 48-50.

[3] Hu Jingyu microstructure and properties of low alloy high strength steel optimized industrial technology 2011 10th publication 45-47.

92