US3215565A

US3215565A - High energy rate processing of ferrous alloys in the metastable austenitic condition

Info

Publication number: US3215565A
Application number: US422073A
Authority: US
Inventors: Richard F Harvey
Original assignee: Individual
Current assignee: Individual
Priority date: 1964-12-24
Filing date: 1964-12-24
Publication date: 1965-11-02
Anticipated expiration: 1982-11-02

Description

Nov. 2, 1965 R. F. HARVEY 3,215,565

HIGH ENERGY RATE PROCESSING OF FERROUS ALLOYS IN THE METAS'I'ABLE AUSTENITIG CONDITION Filed D66. 24, 1964 START ar TRANsF'oRMATmN END OF QSFORMA APPLJGATIQN QF HIGH ENERGY R PROCEDURES H 2 no so so 2 5 IS 30 60 2346810 203060 SECONDS M\NUTES Houns TIME.

IN V EN TOR.

United States Patent HIGH ENERGY RATE PROCESSING OF FERROUS ALLOYS IN THE METASTABLE AUSTENITIC CONDITION Richard F. Harvey, Ross Township, Allegheny County,

Pa. (214 Dombey Drive, Pittsburgh, Pa.) Filed Dec. 24, 1964, Ser. No. 422,073 7 Claims. (Cl. 148-124) This application is a continuation-in-part of my application Serial No. 95,977, filed March 15, 1961, now abandoned, for High Energy Rate Forming and Heat Treating Ferrous Alloys in the Metastable Austenitic Condition.

The present invention relates to the application of high energy rate procedures, such as high pressure by the initiation of explosion, to ferrous alloys in the metastable austenitic condition. More particularly this invention relates to the application of high energy rate procedures which involve the sudden or very rapid application of energy to ferrous alloys in the metastable austenitic; condition for hardening and/or forming. This invention further relates to a process for readily obtaining higher deformations and strengths than the methods of the prior art.

High energy rate processes involve the very rapid or sudden release of energy or in other words involve the application of relatively high pressure at very high speed. Pressures up to about 4,000,000 lbs. per sq. in. are obtainable with high explosives. Generally the deformation is accomplished in less than about 0.001 second and the speed of the material deformation is about 50 to 200 ft. per second as compared with only about 1 to ft. per second for conventional mechanical processes including forging extruding, swaging, peening, drawing, etc. Such conventional mechanical processes will be referred to as low velocity processes in the specification.

The displacement velocity is the actual velocity of the displaced metal and this should not be confused with the rate of energy release which in the case of an explosion is referred to as the detonation velocity. This may be up to about 40,000 ft. per second for high explosives.

Also the critical impact velocity is an important consideration for forming because it limits the maximum strain rate at which material exhibits some ductility.

For a further discussion of high energy rate processes reference is made to the following: A Guide to the Literature on High Velocity Metalworking D.M.I.C. Report 179 Dec. 3, 1962, the Defense Metals Information Center, Battelle Memorial Institute, Columbus 1, Ohio.

This information submitted concerning the effects of high energy rate procedures is known and is included to provide a better basis for the understanding of the present invention.

My investigations show that the application of high energy rate procedures such as explosive shocking to the hardening and forming of ferrous alloys in the metastable austenitic condition in accordance with the teachings of this invention results in improved physical properties together with excellent forming characteristics not obtainable by the methods of the prior art.

It is realized that the prior art discloses mechanical working of metastable austenite and many investigations are reported in the literature starting with the work of the applicant which led to the granting of US. Patent No. 2,717,846.

3,215,565 Patented Nov. 2, 1965 In a later investigation of working metastable austenite D. J. Schmatz et al. in US. Patent No. 2,934,463 reports deformation over in the metastable austenitic condition. However, there are many difficulties in attempting severe deformation using the conventional low velocity methods of working. For example, the equipment including bearings, dies, and machine components do not stand up under such severe conditions and severe deformations cannot readily and inexpensively be accomplished as a commercial practice with the methods of the prior art. The deficiencies and difiiculties with deforming metastable austenite by the methods of the prior art are recognized in a recent Department of Defense publication entitled Summary of Recommendations for Research and Development in Materials, July 1, 1961. This is available from the Office of Technical Services, Department of Commerce as P.B. 161,865. The report recognizes the potential usefulness of thermal mechanical processing but recommends future work in changes in the manner of working.

The difliculties encountered in attempting to obtain high deformations is further exemplified by Technical Documentary Report No. ASD-TR-61-428, March 1962, by R. P. Sernka et a1. Fracturing of H 11 occurred on open die forgineg H 11 steel in the metastable austenitic condition with deformations over 45% and the maximum deformation obtainable for this grade was 73%.

The present invention is designed to meet the needs of defense and aerospace requirements for high strengths and it provides a new and improved commercially feasible method of working metastable austenite to produce unexpected and highly desirable results not disclosed by the methods of the prior art.

A principal objective of this invention is to provide a new method of hardening and/ or forming ferrous metals in the metastable austenitic condition which. is characterized by good formability and higher physical properties than is obtainable by the methods of the prior art.

A further object of this invention is to provide a new method of deep hardening which permits higher deforma tions than have heretofore been obtainable. These high degrees of deformation can be readily accomplished by the teachings of this invention in commercial practice without undue damage to and problems with the deforming equipment which is a charcteristic limitation of methods of the prior art.

A further object of this invention is to provide a new method of hardening and forming where the critical impact velocity is high to permit greater ease of forming.

A further object of this invention is to provide a new method of hardening and forming which results in high strength weight ratios.

Other objects will be apparent from the description which follows. The application of high energy rate methods to ferrous alloys in the metastable austenitic condition results in excellent formability. Also this treatment results in greater conversion of metastable austenite to martensite than would occur without the application of high pressures at high speeds. A higher hardness of at least 2 points Rockwell C may be obtained on A.I.S.I. H 11 tool steel which is used rather widely for aircraft applications. Substantially all of the austenite is converted to martensite on subsequent cooling to room temperature with the result that very excellent physical properties are obtained. The new forming and hardening technique is termed Blastforming.

This method of processing has since been investigated by others with highly successful results. For example P. C. Johnson and B. C. Stern, in a paper delivered before the 92nd A.I.M.E. meeting in Dallas, Texas, reported a substantial improvement in the tensile and yield properties for H 11 and D6AC steels which were explosively hardened in thesubcritical austenitic condition. Another recent investigation made under Air Force contract AF 33 (616) 8191 reported successful results for steels explosively shocked in the metastable austenitic condition.

According to one method of applying the principles of the present invention, the forming and hardening of ferrous metal in the metastable austenitic condition by high energy rate methods such as explosive shock may be completed in four steps which are illustrated schematically with reference to the transformation curves in the drawing. The processing of an air hardening tool steel commonly classified as A.I.S.I. H 11 is illustrated. H 11 is used for high temperature and aircraft applications and has the following typical analysis:

Percent Carbon 0.40 Manganese 0.40 Silicon 1.00 Chromium 5.00 Molybdenum 1.40 Vanadium 0.50

Balance, substantially iron.

The transformation curves show the times required for the austenite to start and to complete transformation at each temperature. Temperature in F. is plotted as the ordinate and time on a loragithmic scale is plotted as the abscissa.

The temperature at which martensite starts to form on cooling is known as the M temperature and is about 520 F. for H 11 steel.

After the H 11 steel is heated above its critical temperature to render it austenitic, it may then be cooled at a rate exceeding its critical cooling rate to a temperature above the M temperature but below the nose of the transformation curve where pearlite starts to form isothermally at about 1450 F.

In the so-called bay region above the M temperature of 520 F. for H 11 steel and below the nose at about 1450 F the steel may be maintained for predetermined lengths of time in the subcritical or metastable austenitic condition. At 800 F., for example, the H 11 steel may be held for several hours before the austenite starts to transform isothermally t Bainite.

In the preferred method of carrying out the principles of the present invention, four steps are involved which are illustrated schematically by lines 1-2, 2-3, 3-4, and 4-5 with relation to the transformation curves in the drawing. Line 12 represents quenching the austenitized steel from above its critical temperature range at a rate at least equal to its critical cooling rate to a suitable temperature in the bay region above its M temperature. The line 2-3 represents holding the steel in the subcritical, austenitic condition at about 800 F. for a length of time to uniformly attain that temperature but insufficiently long to permit substantial transformation to Bainite. While it is preferable to hold the steel as represented by line 2-3 this is not essential and satisfactory results are obtained by shocking the steel in the metastable austenitic condition on cooling through the metastable austenitic range. Line 3-4 represents the application of high energy rate methodssuch as explosive shock to the steel in the metastable austenitic condition. It will be understood that the application of high velocity energy occurs very rapidly, generally in less than 0.001 second and the length of line 3-4 in the drawing appears disproportionately long for purposes of illustration. Line 45 represents cooling the steel generally in air to room temperature.

Molten salts or hot oil may be used as the interrupted quenching medium and these fluids may also be used as the transfer medium for transmitting the shock energy to the steel in the metastable condition.

For high energy rate systems such as explosive and electrical discharge in liquids, the mass contribution is small and the velocity contribution is large towards kinetic energy.

For submerged salt, explosive processing, the weight of the moving salt front is about 5 lbs. and the impact velocity is about 700 ft. per second. Therefore the kinetic energy:- /2 (W/g)v /2 (5/32.2)'(7O0 ):37.980 ft. lbs.

The ratio of the velocity component (v to the mass component (W/g) is about 3.2 10 By comparison it can be shown that for low velocity processing such as with a drop hammer this ratio is about 1.2.

C. W. Marschall in D.M.I.C. Report 192 entitled Hot- Cold Working of Steel to Improve Strength, October 11, 1963, concludes that the degree of strengthening in working steels in the metastable austenitic condition is approximately proportional to the amount of deformation. My investigations on explosively shocked H 11 steel in the subcritical austenitic condition using salt as the transfer medium indicate a deformation of Also the increase in hardness as a result of this processing is 2 points Rockwell C.

A deformation of 95% is higher than the results of methods of the prior art and it should be emphasized that the present invention avoids the mechanical difficulties to bearings, dies, mechanical components, etc., when high deformation degrees are attempted with low velocity methods. It will be noted also there are no mechanical parts involved in explosive shocking of steels in the metastable austenitic condition and this factor may explain in part why higher deformations can be obtained and why high degrees of deformation can be obtained relatively without difiiculty.

While severe deformations have been reported with low velocity working of metastable austenite, the problems involved in attempting high deformations in commercial practice are formidable. The present invention eliminates a major difiiculty encountered with high degrees of deformation as attempted by the methods of the prior art and the present invention shows how very high degrees of deformation can be accomplished readily and without undue difiiculty. My investigations also show how higher degrees of deformation can be accomplished than has previously been obtained by the methods of the prior art.

Steels which have been worked in the metastable aus tenitic condition by low velocity processing invariably show marked directional properties both in microstructure and in physical properties.

Particularly it should be noted also that the highest deformations reported for low velocity working of metastable austenite are for rolling where the elongation of working is substantially all in one direction. As will be expected the structures and properties with this method of low velocity working are highly directional and therefore of limited usefulness. My investigations of high velocity working of metastable austenite indicate that severe deformations can be accomplished with greater uniformity of structure and without the marked directional characteristics of the practice of the prior art.

The occurrence of non-martensitic transformation products after deforming metastable austenite with low velocity methods results in lower physical properties and is undesirable. With severe deformations the metal may be heated by the energy of working to raise the temperature to the zone where transformation to pearlite occurs. High velocity working in accordance with the teachings of this invention minimizes this difficulty because the working is accomplished so rapidly that there is insufiicient time for the heat-to be generated. This invention makes it possible to process steels and ferrous alloys which could not be worked by slow velocity methods be= cause of relatively short incubation periods prior to transformation oi because heat generated in working results in undesirable, non-martensitic transformation products.

While I do not wish to be limited by the consequences of a theory and there is always the likelihood that other factors may be advanced as additional investigational work is conducted, I believe that higher deformations are obtainable, readily and without difiiculty by high velocity shocking steels in the subcritical austenitic condition because the critical impact velocity is raised substantially. The critical impact velocity limits the critical strain rate at which a metal exhibits some ductility and this value is below about 250 ft. per second for many annealed steels. Under conditions of high velocity shocking the metastable austenite has a higher critical impact velocity so that formability is increased and high degrees of deformation are accomplished with greater ease. During the high velocity processing of metastable austenite, it is thought also that the regeneration of dislocations at a high rate may also be a factor.

A difiiculty with low velocity working of metastable austenite is that work hardening occurs and limits the amount of deformation which can be accomplished. This is particularly true of forging. With high velocity processing in accordance with the principles of this invention, the deformation of the metastable austenite is substantially completed before work hardening interferes.

Forming can be accomplished along with hardening. For example, a quench hardenable steel tubing may be simultaneously hardened and formed by expanding to the desired shape while confined in a die. Processing is done in the metastable austenitic condition using high explosives which detonate to provide the desired shock wave. Molten salt at about 800 F. may be used in the bore as the transfer medium and the tubing is placed in a mold cavity so that the internal pressure expands the tubing against the mold walls to harden and shape the tubing. Hardening can also be accomplished without forming as in the case of flat sheets of a hardenable alloy steel which may be subjected to an explosive charge in the metastable austenitic condition using a flat die to support the sheets so that hardening occurs without forming.

For hardening by the principles of this invention, the ferrous alloy should be capable of hardening on quenching and should be capable of being rendered in the metastable austenitic condition. For forming it is not necessary that hardening also occur and any ferrous alloy which can be converted to the metastable austenitic condition is suitable for high velocity forming in accordance with the teachings of this invention.

A deformation range of about 70% to 95% is applicable to the processing of ferrous metals in the metastable austenitic condition by high velocity or high energy rate methods and preferably the degree of deformation should be over about 90%. While examples of explosive shocking were cited, other high energy rate methods including electrical discharge and pneumatic-mechanical means which result in a material deformation speed of about 50 to 200 ft. per second are also satisfactory. It will be understood that this invention may be otherwise embodied within the scope of the following claims.

I claim:

1. The method of hardening a quench hardenable ferrous alloy by high velocity deformation of metastable austenite, comprising the steps of heating the ferrous a1- loy above its critical temperature to render it austenitic, cooling the said ferrous alloy at a rate greater than its critical cooling rate to a temperature range below the temperature of pearlite formation but above the temperature of martensite formation, holding the said ferrous alloy in the stated temperature range for a time insufficient to form Bainite, subjecting the ferrous alloy to explosive shocking to result in a displacement velocity of about 50 to 200 ft. per second at a deformation range of about 70% to 95% and finally cooling the said ferrous alloy to 'room temperature.

2. The improved method of hardening a quench hardenable ferrous alloy by deformation of metastable austenite at high velocity which substantially avoids difficulties with dies and deforming equipment which constitute a characteristic limitation of low velocity metal working methods, said improved method comprising the steps of heating the ferrous alloy above its critical temperature to render it austenitic, quenching at a rate in excess of its critical cooling rate to render the said ferrous alloy in the sub-critical, metastable austenitic condition, subjecting the said ferrous alloy to explosive shock ing to result in a displacement velocity of about 50 to 200 ft. per second at a deformation range of about 70 to 95 and finally cooling the said ferrous alloy to room temperature.

3. The improved method of hardening a quench hardenable ferrous alloy by deformation of metastable austenite at high velocities which substantially avoids difiiculties with work hardening which interferes with the accomplishment of high degrees of deformation with low velocity working methods, said improved method comprising the steps of heating the ferrous alloy above its critical temperature to render it austenitic, quenching at a rate in excess of its critical cooling rate to render the said ferrous alloy in the subcritical, metastable austenitic condition, subjecting the said ferrous alloy to high velocity deformation to result in a displacement velocity of about 50 to 200 ft. per second at a deformation range of about about 70% to 95% and finally cooling the said ferrous alloy to room temperature.

4. The improved method of harden-ing a quench hardenable ferrous alloy by deformation of metastable austenite at high velocities to minimize directional properties characteristic of severe deformations with low velocity metal working methods, said improved method comprising the steps of heating the ferrous alloy above its critical temperature to render it austenitic, quenching at a rate in excess of its critical cooling rate to render the said ferrous alloy in the metastable austenitic condi-'' tion, subjecting the said ferrous alloy to explosive shocking to result in a displacement velocity of about 50 to 200 ft. per second at a deformation in excess of about and finally cooling the said ferrous alloy to room temperature.

5. The improved method of forming ferrous alloys in the metastable austenitic condition at high velocities which comprises the steps of heating the said ferrous alloy above its critical temperature to render it austenitic, cooling the said ferrous alloy at a rate greater than its critical cooling rate to a temperature range below the nose of the transformation curve but above the M temperature to render it in the metastable austenitic condition, subjecting the said ferrous alloy to high energy rate shock at displacement velocities of about 50 to 200 ft. per second to form the said ferrous alloy to the desired shape and finally cooling the said ferrous alloy to room temperature.

6. The improved method of forming a ferrous alloy in the metastable austenitic condition at high velocities which comprises the steps of heating the said ferrous alloy above its critical temperature to render it austenitic,

cooling the said ferrous alloy at a rate in excess of its critical cooling rate to render it in the subcritical, metastable austentitic condition, subjecting the said ferrous alloy to explosive shock at pressures up to 4,000,000 lbs. per square inch in less than about 0.001 second to form the said ferrous alloy to the desired shape, and finally cooling the said ferrous alloy to room temperature, said explosive shock resulting in a critical impact velocity which is substantially higher than the critical impact velocity characteristic of the same ferrous alloy sub jected to low velocity working in the metastable austenitic condition.

7. The improved method of hardening steel by severely 7 deforming metastable austenite ath igh velocities to minimize the formation of undesirable, non-martensitic transformation products which commonly occur With metastable austenite severely deformed by low velocity methods, said improved method comprising the steps of first heating the said steel to above its critical temperature to render it austenitic, cooling at a rate in excess of its critical cooling rate to render it in the subcritica'l, metastable austenitic condition, subjecting the said steel to explosive shock to result in a displacement velocity of about 50 to 200 ft. per second in the said steel in the metastable austenitic condition and finally cooling the said steel to room temperature.

References Cited by the Examiner UNITED STATES PATENTS 9/55 Harvey 148-12.4 4/60 Schmatz et al. 148-12.4

OTHER REFERENCES DAVID L. RECK, Primary Examiner.

Claims

1. THE METHOD OF HARDENING A QUENCH HARDENABLE FERROUS ALLOY BY HIGH VELOCITY DEFORMATION OF METASTABLE AUSTENITE, COMPRISING THE STEPS OF HEATING THE FERROUS ALLOY ABOVE ITS CRITICAL TEMPERATURE TO RENDER IT AUSTENITIC, COOLING THE SAID FERROUS ALLOY AT A RATE GREATER THAN ITS CRITICAL COOLING RATE TO A TEMPERATURE RANGE BELOW THE TEMPERATURE OF PEARLITE FORMATION BUT ABOVE THE TEMPERATURE OF MARTENSITE FORMATION, HOLDING THE SAID FERROUS ALLOY IN THE STATED TEMPERATURE RANGE FOR A TIME INSUFFICIENT TO FORM BAINITE, SUBJECTING THE FERROUS ALLOY TO EXPLOSIVE SHOCKING TO RESULT IN A DISPLACEMENT VELOCITY OF ABOUT 50 TO 200 FT. PER SECOND AT A DEFORMATION RANGE OF ABOUT 70% TO 95% AND FINALLY COOLING THE SAID FERROUS ALLOY TO ROOM TEMPERATURE.