Method for determining hardness limit of austenite class steel

FIELD: measuring equipment.

SUBSTANCE: method includes subjecting samples of steel to preliminary plastic deformation and on basis of wear test results of pre-deformed samples, graph of change of hardness limit of σ-1 samples is built dependent on their level of pre-deformation. Weighed samples are made with same deformation level and value of magnetic tear force P_mag is determined for each weighed sample. Graph of change of magnetic tear force P_mag is built for samples on basis of their pre-deformation level, graph with adjusting curve in coordinates P_mag - σ-1, setting a connection between P_mag and σ-1 dependent on level of pre-deformation. Hardness limit of σ-1 samples is determined by adjusting curve in coordinates P_mag - σ-1.

EFFECT: lower laboriousness, lower costs, higher efficiency.

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The invention relates to measuring technique and can be used to measure fatigue limit austenitic steels subjected to preliminary plastic deformation.

Many details in the food, chemical and petrochemical machine building are made of austenitic steels of the class. In the manufacturing process of these parts is often used plastic deformation.

Plastic deformation has a great impact on the physical and mechanical properties of metals. The austenitic steels are greatly increased strength characteristics (σ_Tthat σ_InHB) and soft, slightly reduced rates of ductility (δ_%; ψ_%).

One of the main causes of loss of efficiency of detail engineering designs, operating in conditions of cyclic stress, is the destruction of their fatigue. To ensure the required resource of the construction with minimal material consumption and the required level of technology components need comprehensive information about the loading conditions and the characteristic fatigue strength of a material is the limit of endurance σ_-1.

There is a method of determining the fatigue limit of the hardness for the two groups of steels - low alloy steels with pearlitic class is for carbon steels (BTW, Markovets Determination of mechanical properties of metals hardness. - M.: Mashinostroenie, 1979 - s-146).

How is that a test is performed to fatigue, the result of which is determined by the limit of endurance of concrete steel. Is determined by the hardness of this steel and then construct a graph of the relationship between the hardness of HB and fatigue limit σ_-1. The middle line in the graphs can be described by the equations: σ_-1=0,1 HB+150 (MPa) for low alloy steels with pearlitic class σ_-1=0,1 HB+100 (MPa) for carbon steels (ibid, p.146).

The disadvantage of this method is its low information content and not enough high accuracy σ_-1depending on HB.

There is a method of determining the fatigue limit of the material in tension-compression in the case of a symmetric cycle.

The method consists in the fact that a lot of steel specimens subjected to axial forces in symmetric cycle (alternating tension and compression) and brought to destruction.

Implemented a way to install specified source (Resistance of materials. Under the General Ed. Academy-4th ed. - Kiev: high school, 1979. S). For implementing the method requires a complex installation, tests are conducted for a long time, leading to huge energy cost, you big to the number of prototypes.

As a prototype of the selected method for determining the fatigue limit of the material in a rotary bending in the case of a symmetric cycle. Implements the specified method at the facility specified in the source (ibid, s).

How is that test specimens should be performed in separate samples of different voltages to detect patterns of change in number of cycles to failure at different stress levels. The resulting experimental data to build a curve.

The disadvantage of this method is the considerable complexity and duration of the tests, high energy costs, as well as the presence of a large number of prototypes.

To reduce the complexity and difficulty of the test, reduce energy costs, reduce the time of testing and reduce the number of samples, the limit of endurance stainless austenitic steels subjected to plastic deformation, are in the following order.

Originally by results of fatigue testing of pre-deformed samples build a graph of the fatigue limit σ_-1samples depending on the extent of their pre-deformation.

Then make samples-sample with the same degree of deformation, each of which is predelut the magnitude of the magnetic force of the separation R _MAGafter which build a graph of magnetic force of the separation R_MAGsamples from the extent of their pre-deformation.

At the final stage of the build graph of the calibration curve in the coordinates of P_MAG-σ_-1establishing the link between R_MAGand σ_-1depending on the degree of prior deformation of the samples.

On the graph with the calibration curve in the coordinates of P_MAG-σ_-1determine the limit of endurance σ_-1samples.

It is known that austenitic stainless steel contain unstable austenite austenite-solution of carbon in γ-gland), γ-iron is not magnetic material (Physical quantities. The Handbook. Edited Isegoria, Esenaliev. M.: Energoatomizdat, 1991; Gulyaev A.P. Metallography, 5th ed., M.: Mashinostroenie, 1977, s-487).

Plastic deformation leads to its partial conversion to α-iron on martensitic mechanism (α-iron has a high magnetic susceptibility). Plastic deformation has a significant impact on the value of the fatigue limit and the magnetic force of the separation began.

We found that the fatigue limit and the magnetic force of a push austenitic steels increases depending on the degree of the preliminary fact deformation is AI, and quite significantly.

Thus, it is possible to directly link the fatigue limit with the magnetic force of the separation of this type of steel.

Figure 1 shows a diagram of an installation for implementing the proposed method.

Figure 2 shows a graph of the fatigue limit at symmetric cycle of loading under pure bending, depending on the degree of prior deformation.

Figure 3 shows a graph of magnetic force detachment austenitic steels depending on the degree of prior deformation.

Figure 4 presents a graph of the calibration curve that establishes the relationship between the fatigue limit and the magnetic force detachment steel depending on its degree of prior deformation.

Installation (figure 1) is an analytical scales, in which the sample-hanging 1 investigated material suspended on a non-magnetic thread 2 in thin-walled Cup 3 of non-magnetic material. The thread is suspended at the end of the rocker arm 4 arrow 5, pivotally mounted on a vertical rack with 6 installed on the scale 7. The rocker has a special adjustable counterweights 8. The rack is mounted on the base 9. On the basis of 9 is, strictly specified under suspension, a permanent magnet 10. At the other end of the rocker arm, also on thread 2, is suspended from the Cup 77, which is located dimensional cargo (sand) 2.

The method is implemented as follows. Namely:

The first stage according to the results of fatigue testing of pre-deformed samples is plotted changes in the endurance limit of the samples depending on the extent of their pre-deformation (figure 2).

The second stage - production samples-hanging with the same degrees of deformation and determining the magnitude of the magnetic force of the cut for each sample at the facility (figure 1) in the following order:

The rocker 4 is balanced by adjustable counterweights 8 after removal from the base 9 of the magnet 10. The balance control is performed by setting the arrow 5 to the zero position on the scale 7. Then put the magnet in the above location and sample-hanging 1 investigated material with prior plastic deformation, is placed in the Cup 3 is provided in the touch position with the magnet rotation of the rocker arm 4. Then podsypanina sand in a Cup 11 until detachment of the sample-sample 1 Cup 3 from the magnet 10. The magnitude of the magnetic force of the separation of the sample-the sample is determined by weighing a measured sand.

The third step is to build the calibration curve in the coordinates of P_MAG-σ_-1(figure 4).

The fatigue limit of the material under investigation are of pre-constructed calibration curve.

Communication between the magnetic force of the separation of steel and the fatigue limit can reduce the number and complexity of tests, that is achieved by eliminating the need for a large number of long-term experiments on determination of the fatigue limit and there is no need to manufacture a large number of accurate samples for testing fatigue.

Validation of the method was carried out on the example of the steel 12X18H10T.

In the first phase samples were made from this steel with different degree of deformation(0; 2; 5; 10; 20%), which was tested in conditions of pure bending symmetric cycle of loading on the machine for fatigue testing of MUI-6000. Test results the dependence of the fatigue limit σ_-1from the degree of deformation of the samples (figure 2).

At the second stage were made samples-sample (sample diameter d=10 mm, height h=15 mm) with the same degree of pre-deformation(0; 2; 5; 10; 20%), and using the setup shown in figure 1, was determined by the magnitude of the magnetic force of the cut for each sample.

The third stage was constructed calibration curve in the coordinates of P_MAG-σ_-1(figure 4).

On the constructed calibration curve can be defined σ_-1steel 12X18H10T with any other (different from 2; 5; 10; 20%) degree of prior deformation.

The proposed method can be used to predict the service life of structures experiencing during the operation of cyclic e.g. the rules and made of stainless austenitic steels (NT, 12X18H10T, etc.), containing in the technology of plastic deformation.

The advantages of the proposed method is simplicity of implementation, low power consumption, small number of samples, reducing the complexity and difficulty of testing the fatigue strength of parts, material which during production were subjected to preliminary plastic deformation.

The method of determining the fatigue limit austenitic steel, which consists in the fact that the samples of this steel is subjected to a preliminary plastic deformation, characterized in that according to the results of fatigue testing of pre-deformed samples build a graph of the fatigue limit σ_-1samples depending on the extent of their pre-deformation, produce samples-sample with the same degree of deformation, determine the magnitude of the magnetic force of the separation R_MAGfor each sample-sample, construct a graph showing the change of the magnetic force of the separation R_MAGsamples from the extent of their pre-deformation, build a graph of the calibration curve in the coordinates of P_MAG-σ_-1establishing the link between R_MAGand σ_-1depending on the degree of prior deformation, determine the limit of endurance σ_-1samples on tarirovannoj the curve in the coordinates of P _MAG-σ_-1.