US3618920A

US3618920A - Thermal processing to improve thermal stress resistance

Info

Publication number: US3618920A
Application number: US20152A
Authority: US
Inventors: Jack R Bohn
Original assignee: TRW Inc
Current assignee: Northrop Grumman Space and Mission Systems Corp
Priority date: 1970-03-16
Filing date: 1970-03-16
Publication date: 1971-11-09
Anticipated expiration: 1988-11-09

Abstract

Tensile thermal stress failure can be avoided by inducing favorable residual stresses which oppose thermal stresses encountered in the duty cycle. Favorable residual stresses are induced by heating an object to ductile temperature, applying a heat flux to localized areas of the object, and cooling so as to retain thermal stresses.

Description

United States Patent Jack R. Bohn Inventor Palos Verdes Penlnlula, Calll. Appl. No. 20,152 Filed Mar. 16, 1970 Patented Nov. 9, 1971 Assignee TRW Inc.

Redondo Beach, Calll.

THERMAL PROCESSING TO IMPROVE THERMAL STRESS RESISTANCE [561 Relerences Cited UNITED STATES PATENTS 1,084.56 l/l9l4 Loss 263/52 Primary Examiner-John J. Camby Attorneys-Daniel T. Anderson, Alan D. Akers and James V.

Tura

ABSTRACT: Tensile thermal stress failure can be avoided by inducing favorable residual stresses which oppose thermal stresses encountered in the duty cycle. Favorable residual stresses are induced by heating an object to ductile temperature, applying a heat flux to localized areas of the object, and cooling so as to retain thennal stresses.

THERMAL PROCESSING TO IMPROVE THERMAL STRESS RESISTANCE SPECIFICATION The invention herein described was made in the course of or under a contract with the U.S. Air Force.

In the past thermal treatment has been employed to improve the structural strength of materials in their duty cycle. The most frequently employed thermal processing is stress relief by annealing. In the annealing process, stresses built into an article during its construction are relieved by heating after the construction has been completed.

in a process analogous to annealing and which improve the structural strength of materials, the present process avoids structural failure due to thermal shock. Tensile thermal shock failure may be avoided by inducing favorable residual stresses which oppose thermal stresses encountered in the duty cycle. Nonsteady state thermal stresses arise as a result of transient temperature gradients in a body and the corresponding differential thermal expansion which cannot be accommodated by geometrically compatible displacements within the body. These stresses continuously adjust themselves in such a way that the internal forces in the body are self equilibrating and the displacements are compatible. if, in the process, either the stresses or the strains reach some critical value, failure may occur. Failure can be defined in a variety of ways depending upon the performance requirements of the component. in general, either fracture or excessive deformation may be taken as a critical failure mode.

The term thermal shock has been used by investigators to describe catastrophic brittle fracture which occurs as a result of high tensile stresses which are generated at the cooler side of transiently heated bodies. The same tensile forces might instead produce excessive deformation in a body if the material were strong enough to resist fracture, or if the material were ductile rather than brittle. Even if the deformation were not excessive during a single heating and cooling cycle, multiple cycling can lead to an accumulated deformation which eventually will become excessive.

Failure may also involve plastic flow and fracture near the heated surface of a body. Post test observations sometimes reveal the presence of checking or cracking in regions near the heated surface where comprehensive plastic flow has occurred during heating. it is here postulated that the reversal of stress at the hot surface, from compressive to tensile, and the reversal of plastic flow from compressive to tensile, occurs not only when the body cools but earlier in the cycle, as soon as the temperature gradient begins to disappear. This will happen even if the overall temperature of the body is still increasing, as might be the case during sustained heating. Thus the material might be put into tension, i.e.. multiaxial tension in most cases, while it is still very hot, and ductile fracture or hot tearing might easily take place. Cracking of this nature could also lead to a loss of material at the hot surface which might be mistaken for compressive spallation or localized erosion in any post test evaluation of a component.

The process according to this invention involves heating a structural material to the point where the material becomes ductile so that if forces were applied it would flow plastically. After the material has reached the temperature of ductility it is subjected to a heat flux locally on the surface of the material by any suitable means such as electron beam induction heating. The temperature gradients employed should be sufficient so as to induce plastic flow of correct magnitude near the heated surface of the material. The proper magnitude of the plastic flow is determined by thermal stress or trial and error experiments. The cooling period is regulated to reduce relaxation of residual stresses. it is desired to retain permanent thermal strain, however forced cooling is not essential to reduce the possibility of relaxation and creep.

One of the chief advantages of this process is that it is applicable to materials of any configuration. Whereas in the past, articles of certain configuration only could be processed prestressed because mechanical restraints which conform to the shape of the article were required, the present method is free of such restrictions. Articles of any configuration may be heated to an elevated temperature, where it is ductile and subjected to a flux to induce permanent thermal strain, and cooled. No mechanical restraint are necessary.

The method which has beendescribed relies on the fact that most refractory and ceramic materials which are weak and brittle at room temperature exhibit plasticity when heated to elevated temperature. Thus in a specific example, if the nozzle insert of a rocket engine is heated slowly to a temperature where the material is ductile and then subjected to a severe shock environment, fracture will not occur, but instance the material will undergo plastic deformation involving compression at the inside diameter and tension at the side diameter. Upon cooling to ambient temperatures, the inside diameter will be in a state of residual tension and the outside diameter will be in residual compression. The magnitude of these residual stresses ,will be influenced by the stress-strain behavior of the material in-tension and compression for the temperature from which the thermal shock processing treatment is initiated, the severity of the subsequent thermal shock treatment, and the cooling rate to ambient. In the application duty cycle the resulting stresses will be lower by the magnitude of the residual stresses which were inducted. Care must be taken during the thermal processing not to induce tensile residual stresses which exceed the fracture strength of the material.

The following table sets forth specific embodiments which further illustrate the invention.

S. Specimen which survived in 3.

b. No prior thermal shock treatment (6.000 B.t.u.lft.' sec.) Pulsed from rm temp to power (4,800 Btu/ft. sec.) Pulled from rm temp to full power (6.000 B.t.u.lft. sec.)

survived without cracking Failed cstsstrophicslly at 0.04 see.

From the above table it can be seen that samples which did not receive the heat flux pulse starting from isothermal conditions at some elevated temperatures could not withstand thermal shock as severe as those which were thermally processed.

I claim:

1. A process for the improvement of thermal shock resistance of an article comprising:

A. heating an article to a temperature where it is ductile B. applying a heat flux to a localized area on thesurface of said article to induce permanent thermal strains throughout the body, and C. cooling said article to retain then'nal stresses.

2. A process for the improvement of thermal shock resistance of an article comprising:

A. heating an article to its ductile temperature,

B. applying a heat flux to localized areas on the surface of I said article so as to induce a predetermined plastic flow near said surface, and

C. cooling said article at a predetermined schedule effective to retain thermal stresses.

3. A process according to claim 2 wherein the heat flux applied is in the magnitude of the heat which the article will experience in the application duty cycle. 5

Claims

1. A process for the improvement of thermal shock resistance of an article comprising: A. heating an article to a temperature where it is ductile B. applying a heat flux to a localized area on the surface of said article to induce permanent thermal strains throughout the body, and C. cooling said article to retain thermal stresses.

2. A process for the improvement of thermal shock resistance of an article comprising: A. heating an article to its ductile temperature, B. applying a heat flux to localized areas on the surface of said article so as to induce a predetermined plastic flow near said surface, and C. cooling said article at a predetermined schedule effective to retain thermal stresses.

3. A process according to claim 2 wherein the heat flux applied is in the magnitude of the heat which the article will experience in the application duty cycle.