KR810000408B1 - Method for providing strong tough metal alloys - Google Patents

Abstract

The wire(12) or strip of this method is deformed at a strain of at least 10% and at a temp. in the range of Md plus 50≰C. In such a manner, the wire or strip has a martensite phase of no greater than 10% by volume and an austenite phase of at least 90% by volume. The produced wire or strip is stretched uniaxially at a strain of at least 10% and a temp. no higher than minus 75≰C. So, the wire or strip has a martensite phase of at least 50% by volume and an austenite phase of at least 10% by volume.

Description

How to improve the toughness of metal alloys

1 and 2 are cutaway side views of the device used in the stretching step.

3 and 4 are micrographs of a 2000 times magnification of microscopic tissue crystals of a material.

5 is a diagram of a sample of an alloy used in some embodiments.

6 is a diagram of another specimen used in the tear toughness test in the Examples.

The present invention relates to a process for improving the strength and toughness of various metal alloys, and to the unique microscopic texture properties of the metal alloys used in the process.

The chemical compositions of the metal alloys to which the present invention is directed are well known and include metal alloys listed in the book Steel Products, Stainless and Heat-Resistant Steel, published by the American Iron and Steel Institute (AISI), Washington, DC, 1974. These alloys are at least initially produced as an austenitic which satisfies the condition that the Md temperature does not exceed + 100 ° C and the Ms temperature does not exceed -100 ° C.

Thus, it will be apparent that the AISI sequence numbers 200 and 300 are of interest in the present invention. In addition, the alloys contemplated in the present invention should be austenitic and have the aforementioned Md and Ms temperatures. Such metals include manganese-substituted non-stainless alloys containing iron, manganese, chromium and carbon, for example X40MnCr18 and X40Mn-Cr22 as the alloys specified in the Deutsche Industrie Norme (DIN) specification. Metals are listed on pages 655 and 656 of the Metal Material Specifications Handbook, published by E FN Spon Ltd, London, 1972.

The term "austenotic" refers to the crystalline microscopic tissue of an alloy, which is called austenetic or austenite when at least 95% of the microscopic tissue has a face-centered cubic structure. It can be said that such alloys are essentially in the austenetic phase. It is convincing that the alloys considered here are essentially in the austenetic or austenite phase at the temperature at which the first deformation step is carried out, irrespective of the work or temperature previously applied. In other words, the metal or alloy to which the first deformation step is applied is annealed before, but is essentially austenitic when the first step is applied.

Another microscopic tissue covered here is the body-centered cubic structure, which is called martensitic or martensite. When at least about 95% of the volume is martensitic, the alloy is considered to be essentially on the martensite phase.

Of course, the microscopic structure can have both an austenite phase and a martensite phase, and the processes discussed in the prior art and according to the present invention change the microscopic structure of the alloy by transforming a portion of the austenite into martensite. to be.

The Md temperature is defined as the temperature at which no martensitic conversion occurs above any temperature, regardless of the amount of mechanical strain applied to the metal or alloy, and is determined by a simple conventional tensile test performed at various temperatures.

The Ms temperature is defined as the temperature at which martensitic transitions begin to occur without mechanical deformation. Ms temperature is also determined by conventional tests.

Some examples of Md temperatures are as follows.

301,302,304,304L steel has Ms temperature below -196 ℃.

Deformation herein refers to mechanical deformation, which occurs in the plastic deformation zone and the plastic deformation zone involves the elastic deformation zone. Deformation is achieved by applying a strain to the material that is above the elastic limit sufficient to change the shape of some or all of the workpiece.

Physical properties related to the present invention include physical properties of strength and toughness. Strength properties are easily determined by simple uniaxial tensile testing as specified in ASTM Standard E-8. This method is described in Part 10 of the 1975 ASTM Standards Yearbook published by the Philadelphia American Society for Testing and Materials (ASTM). The results of this experiment are summarized into three categories: yield strength, tensile strength, and total elongation. In other words, a) Yield strength is a deformation force in which a material exhibits a specific system deviation from the proportional relationship between strain and strain.

The limit deviation here is determined by the offset method of 0.2% strain. d) Tensile strength is the maximum tensile strain at which a material can maintain its original shape. Tensile strength is the ratio of the original cross-sectional area of the workpiece to the load during the tensile test until the workpiece breaks. c) The total elongation is the length of the gauge phase extended until the workpiece breaks in the tensile test, expressed as a percentage of the length of the original gauge phase. As the yield strength and tensile strength of metals increase through metallurgical processes, the total elongation usually decreases.

It is important not only to have a sufficiently high yield strength and tensile strength, but also to have adequate resistance to brittleness in order to satisfy the use where the material requires a highly deformed structure.

With this in mind, metallurgical considerations suggest that sharp blows can concentrate deformation forces on many cracks in the material to determine whether the material is ductile or brittle.

The fracture toughness of the material is a measure of the resistance to brittleness of the material during sharp strikes. ASTM specification E-399 specifies a method for determining the burst toughness of metallic materials by compression test of grooved, fatigue cracked workpieces.

The result is represented by the stress factor Kc, which is a measure of the stress near the tip of the fatigue crack under conditions for observing the onset of crack progression, with large Kc values indicating good burst toughness.

Valuable information can be obtained from the appearance of the rupture surface, where the surface is inclined when the rupture mode is soft and flat when it is brittle. The tear toughness of the roll plate metal depends on how the crack progresses in the rolling direction.

In this specification, the ASTM E-399 method is used to indicate crack plane placement.

The shape of the material of the prior art and the present invention is not a problem. Any shape material that can be handled by conventional techniques, such as flat plates, sheets, strips, thin films, bars, wires, rods, blooms, slabs, can be used.

It is known that the low temperature deformation of the metals and alloys described above (about -196 ° C) significantly improves the tensile strength of the material. In other words, when the AISI 304 stainless steel, which was annealed without conventional aging, was subjected to low temperature deformation at −196 ° C., a tensile strength of 240,000 psi (1,654 Mpa) was obtained. Strength could be obtained. This is a significant improvement for heat treated AISI 304 stainless steel with a tensile strength of about 84,000 psi. Analysis of the aged, cold-deformed stainless steel found that the microscope structure was essentially martensite. It has been found that the high-temperature deformation technique improves tensile strength while reducing the ductility by increasing the martensite ratio, thereby increasing the brittleness and weakening the toughness.

Thus these two properties are relative to each other. A pressure tube is an example that requires a combination of high strength and toughness. High strength materials can make economically desirable light tubes. However, such conduits should not break when air explodes. Therefore, a material with high burst toughness is needed.

Another example is a coil spring that requires a material that is insensitive to rupture and has a high strength to improve fatigue life.

The prior art has led to the conclusion that improvements are needed because many applications require the combination of high strength-toughness with consideration of strength and toughness for low temperature deformed alloys. Therefore, it is an object of the present invention to improve the known low temperature deformation process so that strength as large as that of the prior art can be obtained and at the same time greater toughness than that of the prior art.

Still other objects and advantages are apparent from the following description.

According to the present invention, not only the tensile strength obtained by the conventional low temperature deformation process can be improved, but also high toughness can be obtained, and the non-stainless steel alloy including AISI 200 and 300 stainless steel alloys and iron, manganese, chromium, carbon, etc. Using an austenitic metal alloy selected from the group consisting of, the alloy described above has an Md temperature not higher than 100 ° C. and an Ms temperature no higher than −100 ° C. and includes the following steps.

a. At least 10% of the material is martensitic and at least 90% is orustenite, so that the alloy is deformed with a strain of at least about 10% in the range of ± 50 ° C of the Md temperature (where Md temperature is the Say.)

b. The metal produced in step a) is modified to have at least about 10% strain and at least -50 ° C martensite and at least 10% orstenite phase.

The strain applied in step a) is sometimes referred to herein as 'prestrain', whereas the strain applied in step b) is simply referred to as strain or second stage strain.

Final optimization of strength is achieved by conventional aging of the metal alloy in the range of about 350 ° C-450 ° C.

Analyzing the strained steel alloy of AISI serial number 300 processed by the process of the present invention, including the aging process, using conventional analytical techniques such as X-ray diffraction and electron microscopy, a crystalline microscope prepared by processing It can be observed that the tissue has the same compound as in the initial use, at least 50% is in the martensite phase and at least 10% in the austenite phase. In addition, the tensile strength of the alloy containing the martensite of 50% of the microscopic structure is 190,000 Poond / inch ₂ or more, the tensile strength per 1% increase is increased by more than 2000Pound / inch ₂ at 50% or more.

A particular drink of the present invention is a method of improving the tensile strength and toughness of wires or strips, the wires and strips of which are essentially AISI series having an Md temperature of less than 100 ° C and an Ms temperature of less than -100 ° C. It is composed of an austenitic stainless steel alloy and undergoes the following process steps.

a) The wire or strip is strained in the range of at least about 10% strain and Md -50 ° C to have at least 10% martensite phase and at least 90% austenite phase.

b) The wire or strip is uniaxially stretched at least 10% strain and at temperatures below -75 ° C to have at least 50% martensite phase and at least 10% austenite phase.

Next, it is desirable to optimize the sensitivity by conventional aging treatment of the wire and strip extended at a temperature of 350 ℃-450 ℃.

The alloy used in this process is a conventional alloy as described above. The only condition to be met is to satisfy the definition of austenitic when the first deformation step is applied, the Md temperature should be below 100 ℃, and the Ms temperature should be -100 ℃. Is that.

The deformation is mechanical and takes place in the plastic deformation zone.

The present mechanical deformation technique, which can be used in the first and second deformation processes, is as commercially available as the apparatus for performing this technique.

For example, low ring, forging, elongation, drawing, spinning, bending, swaging, hydroforming, roll forming and the like.

It is clear to those familiar with metallurgical techniques which devices should be used for a variety of techniques, from simple tensile strains to the most complex mechanical strains.

Of course, the deformation process should be sufficient to provide the aforementioned ratios of martensite and austenite, which ratios can first be known by conventional analytical techniques such as X-ray diffraction or magnetic field measurements. It can also be seen based on the operator's experience of modifying various alloys in the range. In order to define the deformation more precisely, it is explained in terms of strain.

The strain generated during the deformation process is much more complex than the strain generated during the simple tensile test, but the reinforcing effect generated during the complex deformation process in the material to which the present invention is applied can be estimated from the reinforcing effect observed during the simple tensile test. Tensile testing ge It uses the principle of "equal uniaxial" or "effective" strains, as described in Dieter's book, "Mechanical Metallurgy."

The minimum strain in the first deformation stage is at least about 10%. At some point there is no upper limit for the percent strain, except for the practical limitation that the microscopic structure changes and the intensity-toughness is minimal. Of course, there are limits on the destruction of materials. In any case the range of strain applied in the first stage is about 10-80%, preferably 20-60%.

As noted, more than 95% of the initial alloys used in this process are austenite and the rest are martensite. During the first step of deformation, the microscopic structure of the alloy changes slowly, with 0-10% on the martensite phase and 90-100% on the austenite phase. It is preferable that 0-5% is a martensite phase and 95-100% is an austenite phase.

Prestrain (Prestrain: strain applied in the first step) is carried out in the range of Md-50 ℃-Md + 50 ℃, Md temperature is the temperature of the alloy used in the deformation step, when the Md temperature is 43 ℃, Md-50 ° C is -7 ° C, and Md + 50 ° C is 93 ° C.

The alloy discussed here changes as described above in the deformation step but is very stable at the temperature of the first deformation step described above. In other words, it is austenically stable.

The second deformation step is similar to the first deformation step in terms of deformation or strain. Sufficient strain must be applied to provide a ratio of martensite to austenite, determined based on conventional analytical techniques and operator experience. The minimum strain applied in the second deformation stage is at least 10%. Again, there is no upper limit on the percent strain, except for the substantial limitation that the microstructure changes and the strength-toughness is minimal, and there is a limit on the bursting of the material. The range of applied strain is 10-60%, with 20-40% being preferred.

It will be readily understood by those skilled in the art that 10% or more strain required in the first and second deformation stages refers to strain applied to all or any part of the workpiece. It will only be found in the range where strain is applied. This is especially important when dealing with complex shapes. For example, pressure vessels, cylinders with discontinuities in welds, various workpieces with discontinuities or defects in design and construction, and workpieces with design and structural defects that locally concentrate stress on any part of the workpiece. . In this case, if less than about 10% and less than about 2-30% of the strain is applied to the entire workpiece, more than about 10% of the strain is discontinued in the discontinuous and bonded portions, so that the strain can be concentrated in the required portion. Thus, it is possible to improve the most vulnerable part of the workpiece in order to have uniform physical properties throughout the workpiece. Thus, if more than about 10% of the strain is applied to the entire workpiece, higher strains will be induced in this discontinuity.

The temperature at which the second stage transformation is carried out is below -75 ° C, preferably below -100 ° C. These temperatures include liquid nitrogen (BP -196 ° C), liquid oxygen (BP -183 ° C), liquid argon (BP -186 ° C), liquid neon (BP -246 ° C), liquid hydrogen (BP -252 ° C), and the like. By performing in a liquid. Liquid nitrogen is preferred. A mixture of dry ice and methanol or ethanol or acetone can also be used to have a boiling point of about -79 ° C. The lower the temperature, the smaller the strain needed to improve the tensile strength. Note that the deformation raises the temperature by applying energy to the material, which eventually rises above 75 ° C. However, if the conditions of the second stage variant are carried out before the temperature rises, this will not affect the present process.

In addition, by performing the cooling operation to -75 ° C before or at the same time, the process is performed faster and more economically. In the second deformation step, the microscopic structure of the metal or alloy is significantly changed such that at least 50% of the volume becomes the martensite phase and at least 10% becomes the austenite phase. It is preferred that 60-90% is martensite and 10-40% is austenite.

The higher the austenite content, the better the toughness of the material.

In the present specification, the microstructure of the product in the initial alloy, prestrain, low temperature deformation, and aging process was considered to be composed of austenite and martensite at the aforementioned ratios.

The presence of any other phase is not considered here. Because such phases are less than 1%, they have no effect on the properties of the alloy.

It should be noted that the range of strain ratios applied in the first and second stages overlaps. Although the ratio is the same, the ratio of prestrain to second stage strain is preferably in the range of about 1: 1 to 3: 1.

After the second step is finished, the alloy is preferably aged to optimize strength. Aging is carried out at a temperature of 350 ° C.-450 ° C. according to conventional methods, preferably in the range of 375 ° C.-425 ° C. Aging time is about 30 minutes-10 hours, and 30 minutes-2.5 hours are preferable. Conventional tests are used here to determine the temperature and time to obtain the best tensile and yield strengths.

Aging improves yield strength rather than tensile strength, and the alloy reaches the highest strength level in that yield strength is approximately equal to tensile strength. It has been pointed out earlier that the application of the process of the present invention to AISI series 300 stainless steel alloys yields a new and unique microscope structure.

When the microscope structure is aging, originally composed of at least 50% martensite and at least 10% austenite, and the microscope structure contains 50% martensite, the alloy has a tensile strength of 190,000 pounds / inch ² . Have Then, the tensile strength is to increase the martensite 2000 pounds / inch ² or ^higher, at least 50% per increase in the 1%.

(190,000 Psi = 1,3000 Mpa, 2000 Psi = 14 Mpa).

The aged microscope structure consists essentially of at least 60% martensite and at least 10% austenite, and the alloy has a tensile strength of 210,000 PSi-260,000PSi if the microscope structure contains 60% martensite, If the structure contains 90% martensite, the tensile strength is 270,000-325,000 PSi (2,239 Mpa).

3 is a light micrograph at 2000 times magnification of the microscope structure treated by the present invention. The alloy is AISI 302. After conventional annealing, the steel is strained to 20% strain at room temperature, followed by 20% strain at -196 ° C and finally aged at 400 ° C for 1.5 hours. The martensitic content is about 25% of the volume.

4 is a light micrograph showing a 2000 times magnification of a microscope structure processed by a conventional low temperature deformation technique. The alloy is AISI 302.

After conventional annealed, 20% strain was strained at -196 ° C and aged at 400 ° C for 1.5 hours. The martensite content is about 25%.

Those who are familiar with metallurgy will find structural differences between the microscope structure obtained by the present invention (Fig. 3) and the microscope structure obtained by the prior art (Fig. 4). Martensitic laths obtained by the present invention are generally short, curved and sometimes exhibit a dendritic appearance.

On the other hand, in Fig. 4, the martensite rod is long, straight, and forms a cross band along the crystal direction.

The most important properties, though dependent on the microscope structure, are not entirely determined by the structure, but rather by the important properties associated with the microscope structure. In other words, the tensile strength corresponding to each% of martensite in the alloy is higher than what is known so far.

As noted above, the features of the present invention and the particular application of the process relate to methods for improving the tensile strength and toughness of wires or strips, the components of which are essentially composed of an austenitic stainless steel alloy of AISI 300. The alloy has a Md temperature of 100 ° C. or less and a Ms temperature of −100 ° C. or less. And the following steps are included.

a. The wires and strips are strained at 10% strain and Md-50 ° C Md + 50 ° C to have at least 10% martensite and 90% austenite.

b. The wires and strips are stretched to at least 10% strain and at temperatures below -75 ° C to have at least 50% martensite and at least 10% austenite.

Tensile strength is optimized by conventional aging of wires and strips elongated at temperatures of 350 ° C-550 ° C.

The above process can be applied to the stretching process but will not be repeated. However, I would like to point out the following points.

Stretching processes, including prestrain and cold deformation, improve the stretching of wires and strips at low temperatures. The advantage is that tensile strength ultimately provides greater tensile strength, regardless of radius and thickness, than cold drawing and low ring, which are related to the radius of the wire and the thickness of the strip, without the need for lubrication and improved torsional yield. do. Stretching is defined as the deformation of a workpiece by one dimension, called the longitudinal direction, much longer than the other two dimensions. For example, wires or strips.

Extensional strain exerts a force in the longitudinal direction such that the entire cross-sectional area of the workpiece is subjected to uniform uniaxial tensile stress during the deformation process. The tensile stress is large enough to cause permanent plastic deformation of the workpiece. The application of stress is expressed in percent strain. Since the term elongation is distinguished from other deformation processes, such as drawing and rowing, which stress on multiple axes, the term uniaxial elongation is used to further emphasize that difference. The skilled person is well aware that drawing through the die produces not only longitudinal tensile stress but also compressive stress in the vertical direction.

Two types of material are of particular interest in the instantaneous stretching process, such as wire and strip.

The second stage process described herein emphasizes the importance of uniaxial stretching and points out that the workpiece is a non-drawing, non-rolling process that excludes the technique of non-uniformly strengthening the workpiece. It was. In the drawing and tearing process, the skin part is strongly strengthened while the core part is weakly strengthened so that the tensile strength of the drawn wire and the drawn strip is limited to the magnitude of the strength where the skin part is not destroyed. This drawback of the drawing wire is even more problematic in special cases where formability is important, such as coil springs. In this case, the skin portion must be flexible enough to be wound around a rod of radius approximately equal to the radius of the wire without breaking. Unfortunately, selective work hardening of the epidermis during the draw process makes the epidermis more rigid and consequently reduces the forming force.

Cold extension processes have been shown to improve tensile strength, formability, torsional properties, fatigue properties, and the like. The prestrain step improves the tensile strength and toughness of the wires and strips, making these materials suitable for normal use.

Like the general process described above, this process may be performed using a conventional apparatus. The first stage deformation process is carried out by conventional drawing and rowing in a specific temperature range under a specific strain. Of course, the wires and strips should be intrinsically austenitic and annealed. In addition to drawing and rowing, other types of modifications may be applied. The first stage of deformation does not require a specific tensile strength. In any case, the materials, strain and temperature are limited.

The second stage transformation process should be carried out in the above-mentioned range of temperatures, i.e., below -75 ° C. And in order to obtain all the advantages of the present invention it must be carried out by an extension process. Otherwise, conventional techniques and apparatus can be used at this stage.

When the wire is working, one form of the apparatus used to perform the second stage stretching process and the related process are described with reference to FIGS. 1 and 2. The process is carried out in an insulated tank 10 filled with a low temperature liquid, such as liquid nitrogen, to some level H. The amount of liquid should be sufficient to completely cover the stretching operation.

The prestrained wire 12 is fed into the tank 10 from the supply spool 13 and passes around the pair of capstans 14 and 15. The capstan is disposed below the liquid surface in the tank 10. The two capstans are identical and consist of two cylindrical rolls of different radius.

The cross section of the capstan 14 cut along the line 2-2 of FIG. 1 is illustrated in FIG. The wires are then guided into the grooves to prevent walking.

The outer groove of the roll 16 is furthest away from the roll 17, and the inner groove of the roll 16 corresponds to the groove adjacent to the roll 17. And, the outer groove of the rool 17 corresponds to the groove in contact with the roll 16, the outer groove of the roll 17 corresponds to the groove furthest from the rool 16. The diameter of the small roll is denoted by D ₀ and the diameter of the large roll is denoted by D ₁ . After injecting the low temperature fluid, the wire 12 is gradually cooled to the temperature of the low temperature fluid and transferred along the outer groove of the roll 16 of the capstan 14 in the direction of the arrow to the outside of the roll 18 of the capstan 15. The process of passing through and returning back to the groove continues between the roll 16 and the roll 18 and gradually moves to the inner groove. The attraction to the wire 12 builds up through friction until the wire reaches point B on the inner groove of the roll 18. Point B passes through point C on the inner groove of the roll 17 of the capstan 14. Both capstans rotate at the same angular speed, resulting in uniform stretching.

The amount of elongation is _{equal to} D ₁ -D ₀ / D ₀ . After point C, the wire continues between the rolls 17 and 19 in the same way as between the rolls 16 and 18 and gradually moves to the outer groove, reducing the attractive force. After passing through the outer groove of the roll 19, the wire 12 leaves the tank 10 and is wound on a reel 21.

The following examples illustrate the invention (in the examples all workpieces contain at least 95% austenite before the first stage deformation process and at least 90% austenite before the low temperature deformation process).

[Example 1-31]

An annealed AISI type 304 stainless steel sheet is used. The chemical composition is as follows.

The anneal is performed by conventional techniques of heating the material to 980 ° C.-1150 ° C. to rapidly cool it.

All sheets are cut into strips 12 inches wide by 0.06 inches thick. The workpiece is cut along a tensile axis parallel to the sheeting direction of the sheet (ASTM E 8), Figure 5 being a diagram of the workpiece. In FIG. 5, the meanings of the reference signs and the sizes indicated in inches are as follows.

The workpieces are processed at 21 ° C. (prestrain) and −196 ° C. (second stage deformation process) at a speed of about 0.1 inches / minute with a Gilmore Model electrohydraulic testing machine. The load is measured by Gilmore, a 20,000 pound load cell. At 21 ° C., the extension is measured with an Instron G-51-15 strain gauge extensometer with a gauge length of 1 inch. The measured load and magnification are then converted into stress and strain by an analog computer and plotted on an X-Y recorder during the experiment.

Strain at −196 ° C. is determined by comparing the length between the gauge marks before and after deformation.

The process at -196 ° C is carried out in an insulated metal thermos filled with liquid nitrogen so that the entire workpiece is submerged in the liquid.

Aging is carried out in a Linderberg model 59744 furnace in air. It is assumed that surface oxidation of the workpiece during aging does not affect the mechanical properties of the workpiece. The temperature of the full length of the workpiece shall not exceed ± 10 ° C at the temperature already specified.

The percent strains at 21 ° C and -196 ° C, the aging time at 400 ° C and the final properties measured at 21 ° C are shown in Table 1.

TABLE 1

Note: Ksi = 1000Psi

Mpa = megapascals.

Example 1-8 illustrates a prior art without a prestrain process, and 9-31 includes a prestrain process.

Yield strength was expressed in Psi at 0.2% elongation, tensile strength was expressed in Psi, and total elongation was expressed in%. These terms are defined above.

The percentage of martensite per volume is determined by quantitative X-ray rotation techniques. The rest is considered austenite. In addition, impurities other than martensite and austenite do not exceed 1%. Martensite is therefore expressed in%, with the remainder representing% of austenite.

[Example 32-35]

The material used in Example 1-31 is used here too, except that the sheet is 0.172 inches thick. Two large workpieces similar to the workpiece shape of FIG. 5 are cut from the sheet. The reduced gauge portion of the workpiece is 3 inches wide and 8 inches long. The workpiece is shortened at −196 ° C. and aged at 400 ° C. for 1 hour. The stretching direction is parallel to the rolling direction of the sheet. Tensile strength is determined using a pin-loaded workpiece having a 2 inch gauge length by ASTM method E8. The tensile strengths of Examples 32 and 33 for the two workpieces are shown in Table II.

In Examples 34 and 35, two workpieces of annealed 304 stainless steel with a 0.251 inch thick sheet rolled to a thickness of 0.186 inch, which corresponds to a 30% uniaxial strain. The workpiece is rolled back to 0.161 inch thickness at -196 ° C, corresponding to a 16% uniaxial strain.

The tensile strengths of Examples 34 and 35 for each of the measured workpieces are shown in Table II.

The workpieces according to the invention of Examples 34 and 35 contain 67% martensite and 33% orestate, whereas the prior art workpieces of Examples 32 and 33 are 85% martensite and 15% It contains austenite.

To test the fracture toughness of the processed workpiece, a small tensile workpiece was made and aged at 400 ° C. for 1 hour. The shape of the miniature tensile workpiece is illustrated in FIG.

The meaning of the reference numerals in Figure 6 and the measurement per inch are as follows.

Small tension workpieces are oriented such that the burst passage is perpendicular to the stretching or rolling direction (LT). LT refers to the ASTM 399 method for workpiece orientation. The first letter indicates the direction of loading and the second letter indicates the direction of crack progression.

All workpieces have sharp notches due to fatigue pre-crackng. Each workpiece is subjected to tension-tension loading using 10 alternating stresses so that the cracks are 0.125 inches from the notches and 1.125 inches of unbroken ligaments. The strength of the stress used is 65 Ksi inches (71.4 Mpa meters) and the crack growth rate is 2 × 10 ⁻⁵ inches / cycle. The value of R, defined as minimum load versus maximum load, is 0.25.

After precracking, each workpiece is stopped and the rod is recorded as a figure shift function. Cross-head speed is 5 inches / hour. Although a linear variable differential transducer measuring loading pin displacement was used in place of the elongation stiffness clip for measuring crack opening, the process for determining the initial crack progression (ASTM method) E 399).

The stress strength at which crack progression begins is shown in Table II under Crack Toughness. The value is only an estimate because the clip-on elongation stiffness was not used to measure crack opening. Therefore, ke is displayed instead of kc. Table 2 shows the tensile sensitivity and test temperature crack-toughness, crack mode (observed by visual).

TABLE 2

Workpieces 32 and 33 by conventional cold deformation techniques can be compared to work pieces 34 and 35 by the process of the present invention.

Workpieces 32 and 34 can both be compared because they were tested at 25 ° C., and work pieces 33 and 35 were tested at −196 ° C.

In workpiece 32 according to the prior art and workpiece 34 of the present invention, the increase in tensile strength is about 19%, while the decrease in crack toughness is only about 2.8%. It is evident that a significant increase in tensile strength has only subtle nutrition for crack toughness. Therefore, in the prior art, the crack toughness also decreased as the tensile strength increased.

In the conventional work piece 33 and the work piece 35 for the present invention, the increase in tensile strength is only about 8.9, but the increase in cracking property is 67%, and whether it is hard or not, it is partially ductile.

Example 36-44

An annealed AISI type 302 stainless steel wire was used and the chemical composition was as follows.

The annealing used a conventional technique of rapidly cooling anneal to 980 ° -1150 ° C.

The wire is first stretched at 21 ° C. (except for Example 36) with a prestrain of any size in the conventional manner, and then in liquid nitrogen.

All processes and apparatus related to this are described above and illustrated in FIGS. 1 and 2. The wire of each example is aged for 2 hours at 400 ° C. according to the conventional method. Initial wire diameters, percent prestrains at 21 ° C and -196 ° C, and tensile strength are listed in Table 3. The martensite content of the processed wire of each Example is 60% or more (except Example 36).

TABLE 3

Example 45-55

This example relates to tensile strength and torsion yield strength. For example, the torsional yield strength of a wire is a 2% torsional yield strength, which is the 2% torsional yield strength whereby the wire is twisted while increasing its angle, resulting in the first permanent angular distortion. It is defined as the shear stress occurring on the surface of the wire when twisted at an angle sufficient to generate. Similar definitions apply for 5% torsion yield strength.

The torsion yield strength of the wire to be used as a spring is preferably as large as possible with respect to the tensile strength of the wire.

After deformation, all workpieces are conventionally aged at 400 ° C., stretched or draw process at 21 ° C. in the conventional manner, and at −196 ° C. in the process and apparatus described in this specification and figures 1 and 2. By means of a stretching process in liquid nitrogen. The martensite content of all workpieces processed at -196 ° C is at least 60%.

The process at -196 ° C is carried out in an insulated metal thermos filled with liquid nitrogen such that the entire workpiece is submerged in liquid nitrogen. Aging is carried out in Linderberg model 59744 circuit in air. Surface oxidation of the wires occurring during the aging process is considered to have no effect on the mechanical properties of the workpiece.

The temperature change along the length of all workpieces does not deviate by more than ± 10 ° C from the temperature already specified.

The percent volume of martensite is determined by quantitative X-ray diffraction techniques and the rest is considered austenite.

Since impurities are less than 1%, they are not considered here. Tensile tests of all workpieces are performed according to ASTM method E 8, and torsion tests are performed by the method described above.

The wires of Examples 45, 46, 47, 49-54 show a suitable forming force to wind the wire around a rod having a radius equal to the radius of the wire without cracking.

In Examples 45 and 49-54, the strain applied at 21 ° C. was by conventional stretching, in Examples 48 and 55 by conventional drawing at full hard, and in Example 47 By conventional drawing at 1/4 hard.

In all examples except 48 and 55 the strain applied at −196 ° C. is of course by stretching. In Example 46 no strain was applied at 21 ° C. and in Example 48 and 55 no strain was applied at −196 ° C.

The percent strain, final wire diameter tensile strength after aging treatment, torsion yield strength after aging treatment, and the ratio of torsion yield strength and tensile strength are listed in Table 4.

TABLE 4

Claims (1)

In the group consisting of stainless steel alloys of AISI series Nos. 200 and 300 and non-stainless steel grades containing iron, manganese, chromium and carbon, the Md temperature of which is selected does not exceed 100 ° C. and the Ms temperature is − The method for improving the strength and toughness of the austenitic metal alloy not exceeding 100 ℃. The alloy is deformed with a strain of at least 10% at a temperature in which the Md temperature of the alloy to be deformed is in the range of Md-50 ° C to + 50 ° C so that the material is at least 90% Volume percent austenite phase, The resulting material is transformed into a strain of at least 10% at a temperature not exceeding -75 ° C so that the material has at least 50% (% by volume) of martensite phase and at least 10% (% by volume) of austenite phase. Method for improving the toughness of the metal alloy characterized by consisting of steps.

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