METHOD OF STEEL PROCESSING COMBINING THERMAL AND MECHANICAL SURFACE TREATMENT TO CONTROL METALLURGICAL PHASE AND MECHANICAL RESPONSE

Abstract

A method of steel processing combining thermal and mechanical processing of steels in controlled sequences. The method of the present invention combines thermal and mechanical processing in controlled sequences to achieve material property results that are superior to existing methods. The method allows for manipulation of steel processing variables, which promotes further elimination of retained austenite, additional residual compression, reduced surface tension, increased material strength, increased compressive stresses at the surface, and significantly improved bending fatigue and wear resistance. By varying the sequence of mechanical processing of the steel, desired residual compressive stress responses and hardness levels may be achieved. In addition, this processing can reduce embrittlement caused by late stage phase transformation.

Description

BACKGROUND OF THE INVENTION

The present invention relates to metallurgical processing of alloy steels for control of metallurgical phase, mechanical properties, and in service part performance. In particular, the patent pertains to the controlled sequencing of thermal processing (heat treatment) with intermediate and or post mechanical treatments.

In the heat treatment of steel, quench hardening by itself or in combination with carburizing, nitriding, nitrocarburizng, or carbonitriding all have limiting factors that prevent optimal hardening and other desired mechanical responses due to the steels' metallurgy. These factors include, but are not limited to: a) Retained austenite stability; b) carburization response (depth of carbon diffusion); c) formation of intergranular and transgranular carbides; d) residual stress response to the thermal treatment; e) depth of nitiriding; and f) brittleness, hardness of the thermally treated surface layer, among others. In particular, tempering practices designed to control final hardness and strength in the steel may limit service performance in terms of bending fatigue and wear resistance. Thus current processing methods are limited in their ability to affect steel properties. A specific example is the inability to affect hardness, strength, residual compression and bending fatigue life individually. Another example is the ability to eliminate retained austenite without inducing tensile stresses and cracking.

Existing problems with current methods include an inability to eliminate retained austenite, shallow residual compression, residual surface tension, and fatigue life limitations in carburized parts from post heat treatment surface processing such as laser peening and other mechanical treatments.

The patent defines improved metallurgical processes to help alleviate these problems.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a method of treating steel is disclosed. The method includes: heating the steel; applying a quench hardening sequence to the steel; applying a mechanical surface treatment to the steel after the quench hardening; and applying one or more cycles of a deep freeze and a tempering sequence to the steel. Mechanical treatments include the group consisting of shot peening, laser peening, low plasticity burnishing, cavitation peening, vibratory processing, and other methods applying kinetic energy to affect internal strain. The steel may include a non-carburized steel. The steel may also include one of a carburized, a nitrocarburized, a carbonitrided, a nitrided, a precipitation hardened, an induction hardened, a flame hardened, among other steels.

In a preferred embodiment, the mechanical treatment is a laser peening process. In other embodiments, the mechanical treatment is applied immediately following the quench hardening step.

In other aspects of the invention, a method of treating steel includes: heating the steel; applying a quench hardening to the steel to thermally destabilize retained austenite; applying a first cycle of a deep freeze and a tempering sequence to the steel; applying a mechanical treatment to the steel; and applying one or more cycles of a subsequent deep freeze and a subsequent tempering sequence to the steel.

The mechanical treatments include shot peening, laser peening, low plasticity burnishing, cavitation peening, vibratory processing, and other methods applying kinetic energy to affect internal strain. In a preferred embodiment, the mechanical treatment is a laser peening process. The steel may be a non-carburized steel. The steel may also include one of a carburized, a nitrocarburized, a carbonitrided, a nitrided, a precipitation hardened, an induction hardened, a flame hardened, among other steels. The mechanical treatment may be applied anytime after the initial quench hardening process.

In yet other aspects of the invention, a method of treating steel includes: heating the steel; applying quench hardening to the steel; applying a first cycle of a deep freeze and a tempering sequence to the steel; applying an intermediate mechanical treatment to the steel after the first cycle; applying a subsequent cycle of the deep freeze and tempering to the steel, after the intermediate mechanical treatment; and then applying a second mechanical treatment sequence to the steel.

In some embodiments, the mechanical treatments include shot peening, laser peening, low plasticity burnishing, cavitation peening, vibratory processing, and other methods applying kinetic energy to affect internal strain. Preferably, the mechanical treatment is applied immediately following each subsequent cycle of the deep freeze and the tempering sequence.

The steel may be a non-carburized steel. The steel may also include one of a carburized, a nitrocarburized, a carbonitrided, a nitrided, a precipitation hardened, an induction hardened, a flame hardened, among other steels. These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of current standard process sequence;

FIG. 2 is a flow diagram of process sequence A;

FIG. 3 is a flow diagram of process sequence B;

FIG. 4 is a flow diagram of process sequence C;

FIG. 5 are photos of carburized and hardened Ferrium S53 steel subjected to final laser peening treatment;

FIG. 6 are photos of carburized Ferrium S53 steel subjected to laser peening treatment prior to tempering; and

FIG. 7 are photos of carburized Ferrium S53 steel subjected to laser peening treatment after the first deep freeze/temper sequence, but before the second deep freeze/temper treatment.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description is of the best currently contemplated modes of carrying out exemplary embodiments of the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention.

Broadly, embodiments of the present invention provide for improved processing techniques for steel, including carburized, a nitrocarburized, a carbonitrided, a nitrided, a precipitation hardened, an induction hardened, or a flame hardened, among other steels. The present invention provides a means for application of combined thermal and mechanical processing of heat treated steel in specific sequences, so as to affect one or more of the following beneficial responses to the alloy steel (including but not limited to): 1) Controlled residual compressive stress depth; 2) Reduction or elimination of retained austenite in the heat treated structure; 3) Precipitation and ordering of alloy carbides prior to and during tempering and the mechanical processing; 4) Enhancement of residual compressive stresses in the softened condition so as to reduce cracking potential during part service loading; 5) Ability to thermally refine (temper) the hardened microstructure obtained by the mechanical treatment; and 6) Maintain a beneficial residual compressive stress state through the part cross section.

The method of the present invention combines thermal and mechanical processing in controlled sequences to achieve material property results that are superior to existing methods. The method allows for manipulation of steel processing variables, which promote further elimination of retained austenite, additional residual compression, reduced surface tension, increased material strength, increased compressive stresses at the surface, and improved bending fatigue and wear resistance.

The drawings of FIG. 1 show a conventional process of heat treatment and mechanical treatment. Currently, a full steel heat treatment 20 is conducted followed by a mechanical surface treatment 30. This is done in an attempt to achieve improved surface compressive stresses. However, inherent metallurgical complexities in the deep freeze 24 and tempering treatment 25 cycles of highly alloyed carburized steels 22 produce a microstructure prone to embrittlement. This embrittlement leads to reduced fracture toughness. The embrittlement and reduced fracture toughness result from “pockets” of untempered martensite transforming from remnants of retained austenite in the carburized case 22. This transformation is due to mechanical processing 30 following completed heat treatment 20.

FIG. 5 shows an example of this structure for carburized and hardened Ferrium S53 steel 22 subjected to a final laser peening treatment 32. It is commonly seen that many carburized, alloy steels 22 subjected to the standard final laser peening process 32 exhibit a reduction in fatigue resistance due to reduced fracture toughness. This reduction in fatigue resistance and reduced fracture toughness is a direct result of the described mechanically induced martensite transformation. Therefore, alternative sequencing is required to achieve both the microstructural integrity and mechanical toughness required in part service. This is especially true for carburized steel 22 prone to retained austenite formation. In the standard process 20, after optional carburization, nitrocarburizing, carbonitriding, nitriding, or other treatments, the selected steel 21, 22 first undergoes a quench hardening step 23. The quench hardening 23 may include gas, oil, water, a salt bath, polymer, etc. The quenched steel may then undergo one or more deep freeze 24 and tempering 25 cycles, followed by a subsequent mechanical processing 30. This mechanical processing 30 may include one or more of a shot peening 31, a laser peening 32, a cavitation peening 33, a low plasticity burnishing 34, other mechanical processes 35 including vibratory processing and other methods applying kinetic energy to affect internal strain.

The best currently contemplated modes of the exemplary embodiments of the invention include but are not limited to three possible variations in thermal / mechanical sequencing.

In a first process of the invention, shown in FIG. 2, the mechanical treatment 30 is conducted immediately after the quench hardening step 23. This is done to achieve maximum retained austenite transformation prior to the deep freeze 24/tempering 25 sequence. FIG. 6. shows carburized S53's microstructure. This was achieved by a laser peening process 32 prior to tempering 25. This microstructure is a highly twinned, high carbon martensite consisting of heavy acicular plates. The inherent retained austenite has been almost fully transformed to martensite. Hardness prior to tempering 25 here is HRC 48. FIG. 6 also shows the final microstructure for carburized Ferrium S53 steel after heat treatment, then being laser peened 32, followed by a double deep freeze 24 and tempering 25 treatment. Here the microstructure has a hardness of HRC 58-61. Note the absence of untempered martensite compared with the structure shown in FIG. 5.

The second embodiment of the invention is the processing sequence referenced in FIG. 3. In this process, the mechanical processing treatment 30 is conducted after a first 24, 25, but before the second (and possible subsequent) deep freeze 24′/temper 25′ treatments. This is done to better maintain residual compressive stress response throughout the process, while achieving a higher hardness due to enhanced secondary carbide precipitation.

The initial deep freeze 24 and temper 25 cycle thermally destabilizes the retained austenite, which is then more readily transformed to the highly twinned, acicular plate martensite. This martensite is then tempered out during the remaining thermal sequencing 24′, 25′. Subsequent deep freezing provides additional destabilizing potential to the remaining retained austenite. The subsequent deep freeze 24′/tempering 25′ steps maintain residual compression, but enhance hardness. FIG. 7 shows an example microstructure for carburized Ferrium S53 steel achieved by this process. The associated hardness is also provided.

In a third embodiment of the invention, shown in FIG. 4, mechanical processing 30 is conducted after the first 24, 25, second 24′, 25′, and any subsequent deep freeze 24″/temper 25″ sequences. In this process sequence, an intermediate mechanical processing 30 achieves retained austenite transformation and deep residual compression. A final mechanical process 30 is used to locally compress and harden the working surface to further enhance wear and bending fatigue performance.

As shown, each of the elements of the steel processing method can be arranged in specific sequences to have specific functional results in the final steel product. The sequencing is key to achieving metallurgical response not currently achievable by conventional means.

In Sequence A (FIG. 2), the process seeks to achieve early stage retained austenite transformation, and introduction of residual compression prior to the deep freeze/tempering sequence designed for secondary hardening.

In Sequence B (FIG. 3), the process seeks to achieve residual compression after the initial secondary hardening during which preliminary austenite destabilization occurs. In this sequencing, embrittling of the structure is reduced.

In Sequence C (FIG. 4), the process uses a final mechanical treatment 30′ to impart local additional increase in residual compression. Non-volumetric shock wave/deformation techniques (i.e. shot peening 31, cavitation peening 34, low plasticity burnishing 33, or other mechanical treatments) may be employed in the final stage 30′ to achieve this objective. Detrimental retained austenite transformation to martensite near the surface will not occur in this final step 30′ as it has been completed in the sequence prior to tempering 25′.

It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.

Claims

1. A method of treating steel, comprising: heating the steel;applying a quench hardening to the steel;applying a mechanical treatment to the steel after the quench hardening; andapplying one or more cycles of a deep freeze and a tempering sequence to the steel.

2. The method of claim 1, wherein the mechanical treatments are selected from the group consisting of shot peening, laser peening, low plasticity burnishing, cavitation peening, and vibratory processing.

3. The method of claim 1, wherein the steel comprises: a non-carburized steel.

4. The method of claim 1, wherein the steel comprises: carburized, a nitrocarburized, a carbonitrided, a nitrided, a precipitation hardened, an induction hardened, and a flame hardened.

5. The method of claim 1, wherein the mechanical treatment is applied immediately following the quench hardening step.

6. A method of treating steel, comprising: heating the steel;applying a quench hardening to the steel to thermally destabilize retained austenite in the steel;applying a first cycle of a deep freeze and a tempering sequence to the steel;applying a mechanical treatment to the steel; andapplying one or more cycles of a subsequent deep freeze and a subsequent tempering sequence to the steel.

7. The method of claim 6, wherein the mechanical treatment is selected from the group consisting of shot peening, laser peening, low plasticity burnishing, cavitation peening, and vibratory processing.

8. The method of claim 6, wherein the steel comprises: a non-carburized steel.

9. The method of claim 6, wherein the steel is selected from the group consisting of a carburized, a nitrocarburized, a carbonitrided, a nitrided, a precipitation hardened, an induction hardened, or a flame hardened steel.

10. The method of claim 6, wherein the mechanical treatment is applied immediately following immediately after the first cycle.

11. A method of treating steel, comprising: heating the steel;applying a quench hardening to the steel;applying a first cycle of a deep freeze and a tempering sequence to the steel;applying an intermediate mechanical treatment to the steel after the first cycle;applying a subsequent cycle of the deep freeze and the tempering sequence to the steel, after the intermediate mechanical treatment; andapplying a second mechanical treatment sequence to the steel.

12. The method of claim 11, wherein the mechanical surface treatment is selected from the group consisting of shot peening, laser peening, low plasticity burnishing, cavitation peening, and vibratory processing.

13. The method of claim 11, wherein the steel comprises: a non-carburized steel.

14. The method of claim 11, wherein the steel is selected from the group consisting of a carburized, a nitrocarburized, a carbonitrided, a nitrided, a precipitation hardened, an induction hardened, or a flame hardened steel.

15. The method of claim 11, wherein the mechanical surface treatment is applied immediately following each subsequent cycle of the deep freeze and the tempering sequence.