Research on the Heat Processing Theory and Damage of Martensitic Stainless Steel

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Abstract: Through the Gleeble-1500D thermal simulator, the paper were carried out hot compression and tensile experiments on the 1Cr13 martensitic stainless steel, and get the different deformation curve under the conditions of stress strain. At the same time, we get the constitutive equations and Z parameters, which provide necessary material model for numerical simulation of the stainless steel forging process. The compression and tensile tests were simulated with DEFORM-3D simulation software, respectively; get the material value of the fracture damage ductile fracture critical damage and material void initiation damage, it will reasonable provide reference in production process and prevention crack in the process of forging.

Keywords: Strength; hardness; 1Cr13 martensitic stainless steel; hot deformation behavior; damage value;

0 Introductions

With the development of the national economy and the improvement of people's living standard, stainless steel because of its corrosion resistance, beautiful appearance and so on, people pay more and more attention to it, it has been widely used in various fields, such as pipes, valves etc.. Production volume is not forged stainless steel parts in China, forging technology level is not high, so in the use of high strength stainless steel production of large and complex parts of the forging process and product quality is not stable, many product should be formed by forging process parts have to adopt casting or rolling type material processing, which result decreased performance and service life. In order to meet the growing needs of the community, the production enterprises adopt the method of parts of the forging instead of casting.

However, in the horizontal forging technology is not very high in forging stainless steel, so in the valve manufacturing after appeared crack and toughness is not qualified. So the paper study problems with the enterprises in the production process, in-depth study of the factors causing the defects, and through the numerical simulation of cracks in the material was predicted on the whole body, and the forming die are studied in forging process, providing evidence for the body shaping.

1 Study on heat deformation behavior of 1Cr13 martensitic stainless steel

The flow stress is a very important concept for the plastic process, so the flow stress of the material studied is a key field of materials processing. If the numerical simulation uses the finite element method, to establish the constitutive model of materials; constitutive relationship is the description of materials at different deformation temperatures, strain rates. In this regard, the accuracy is directly related to the accuracy of simulation results between the materials, the materials were measured, also provides a theoretical basis for optimal forging process materials.

1.1 Hot compression test

Cr13 stainless steel is martensite heat-resistant stainless steel, after quenching and tempering the material has corrosion resistance, heat resistance, impact toughness, and is a kind of hardenable stainless steel. This experiment selected material is 100 mm × 100 mm × 200 mm 1Cr13 martensitic stainless steel block after forging by a forging company, its chemical composition as shown in table 1. Deformation temperature at 850 ～ 1200 ℃, interval are 50 ℃, strain rate were 0.005, 0.05, 0.5 and 2.5s-1, the deformation degree of 50%. The whole test process is caused by the thermal simulation testing machine automatic recording of data compression, by analyzing the function itself can directly obtained be the true stress- true strain curves.

1.2 The rheological curves in different deformation temperature conditions

As shown in Figure 1, it can be obtained at different deformation temperature of martensitic stainless steel 1Cr13 true stress-strain curve. Visible, in the early days of rheological deformation stress and increased rapidly with the increase of the strain, the flow stress reaches a certain value after the rise speed become slowly and tends to a stable value.

As can be seen from the graph, so the true stress-strain curve showed the same trend at low temperature, the deformation at the beginning because of material hardening effects enable the flow stress with deformation increased rapidly, and reached a peak. With the increase of strain capacity, low temperature the true stress did not change significantly, which is characterized by steady flow; however, under the condition of high temperature, the flow stress increases with the increasing of strain after reaching the peak value gradually decreased, the softening phenomenon.

1.3 Different rheological curve in the conditions of variable rate

Plastic deformation is a hardening and softening the contradictory process, as shown in Figure 2 is the 1Cr13 martensitic stainless steel under different stress conditions of rheological curve with variable rate, at the same deformation temperature, the flow stress increases with the increase of strain rate and increase sharply, this is because the increase of strain rate, the metal within no time full of dynamic recovery and recrystallization softening, increased work hardening, deformation resistance increased.

Therefore, the lower strain rate and high deformation temperature will due to the dynamic recovery and fully recrystallization , more softening due to the greater decreases more significant peak stress.

2 Establishment of constitutive equation

The constitutive relation of material descript the material flow in the process of forming stress and thermodynamic parameters, such as the relation of strain, stress the interaction between strain rate and deformation temperature, that is for realizing the precise numerical simulation.

The Z parameter as the hot working parameters of the material in the thermal deformation, its physical meaning is deformation rate factor of temperature compensation, defined as:

(1)

Wherein:εis the strain rate s-1 ;Q is thermal deformation activation energy KJ/mol; R=8.3145 J/(mol・K); T is deformation temperature K; σis flow stress; F(σ) is stress function;αis material constants, α=β/n' Mpa-1

We can describe the relations between the peak stress and strain rate, deformation temperature and the deformation activation energy expressions, that is:

In the low stress level: when ασ

Under high stress level: when ασ>1.2, formula (2) simplified as:ε=A"exp(β・σn)

By means of fitting a straight line with slope, can draw deformation constitutive equation of 1Cr13 martensitic stainless steel under the experimental conditions of thermal:

2.1 Zener-Hollomon parameter

During hot deformation process, to make the material dynamic softening (i.e. dynamic recovery and recrystallization) advantage is material in low strain rate and deformation temperature deformation. The Z parameter is the temperature corrected strain rate, reflects an important parameter affecting the strain rate deformation temperature, thermal processing, activation energy Q is obtained through the above thermal deformation, to calculate the Z parameters of 1Cr13 martensitic stainless steel, hot deformation, that is:

(4)

The formula (4) logarithm gets linear relationship between . With the increase of Z parameters, the corresponding to the peak stress increases. The Z parameter describes the deformation behavior of 1Cr13 stainless steel under certain deformation conditions, namely the Z parameter value is small, the migration of material internal dislocation and grain boundary is higher, deformation occurs in the dynamic recrystallization of the greater tendency;

2.3 The critical damage during hot compression

The material fracture is damage accumulation of the material in the plastic deformation, damage to fracture tendency of reaction materials, namely the material plastic forming process if the damage to be the critical damage factor of a material, that the appearance of cracks. In order to study the metal forging crack resistance and thermal sensitivity, usually hot upsetting test to simulate the hot forging process, in order to ensure the quality of the forging process, we studied the critical damage factor 1Cr13 martensitic stainless steel material.

Usually the introduction of Cockcroft-Latham damage sensitive rate definition. Combined with the definition of damage factor:

The concept of damage sensitive rate: value ratio between damage increment D in unit time increment T and existing cumulative damage D, that is:

Generally, when the material damage sensitive rate R don’t sensitive on the damage, material has reached the critical damage for materials to cracks.

We can see from table 2, when the material critical damage factor as the temperature does not have too big change the critical damage factor in the same strain rate under the condition, which shows the temperature not sensitive to it; when at the same temperature, the critical damage factor with should increase at first and then decreases and strain rate. The critical damage under different deformation conditions of 1Cr13 martensitic stainless steel is in 0.1397 ~ 0.3407 between, if the damage factor in the deformation of materials to meet and exceed the limits, forging it may appear crack.

3 Study on formation of stainless steel high temperature plasticity, crack and the critical damage

The plasticity of metal is the premise of plastic processing, that is to say the metal plastic is better, more with plastic deformation ability, and allows to produce also increased; conversely, if the metal once more power will be immediately broken, makes the plastic processing is not, but in the actual production hope the deformation of the metal has good plasticity.

3.1 Experimental materials and methods

After forging, we machine it into cylinder specimens of φ 10 × 121.5mm. The experimental machine on the tensile experiments on Gleeble-1500D thermal simulation, the temperature range of tensile test: 900 ℃ ~ 1200 ℃, the temperature interval was 50 ℃, strain rate were 0.01s-1, 0.2s-1, 2.5s-1, sample welding thermocouple wire, thermocouple and thermal simulation machine through a connected, purpose is to research on high temperature ductility of the materials. The specimen with heating rate of heating at 10 ℃ /s to 1200 ℃, holding for 2 minutes, and then to 5 ℃ /s cooling rate of cooling to the required tensile temperature, heat 1 minutes.

We were simulated different sample in the different temperature, strain rate tensile specimen conditions, fracture test specimen is from the void initiation, growth, aggregation, and eventually formed a macroscopic crack. From the simulation process to obtain the critical damage in the ductile fracture of the sample value, and the critical damage the value of fracture toughness increases with increasing temperature; if at the same temperature, the fracture critical damage value increases with the increase of strain rate. And the material in the critical damage empty value (critical damage material value Ci), the rule is with the increase of temperature decreased but the downward trend is quite small, indicate that the temperature of the material cavity initiation of critical damage value is relatively small.

4 Conclusions

According to defects (such as cracks) of the 1Cr13 martensitic stainless steel in the process of forging and the performance does not meet the requirements, the paper use thermodynamic and numerical simulation, analysis of the factors causing defective valve by based experiments and forging simulation, to provide technical support and practical reference for the production of valve steel forging.

Reference

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