CA1196257A - Three-step treatment of stainless steels having metastable austenitic and martensitic phases to increase resistance to chloride corrosion - Google Patents
Three-step treatment of stainless steels having metastable austenitic and martensitic phases to increase resistance to chloride corrosionInfo
- Publication number
- CA1196257A CA1196257A CA000402682A CA402682A CA1196257A CA 1196257 A CA1196257 A CA 1196257A CA 000402682 A CA000402682 A CA 000402682A CA 402682 A CA402682 A CA 402682A CA 1196257 A CA1196257 A CA 1196257A
- Authority
- CA
- Canada
- Prior art keywords
- shot peening
- stainless steel
- intensity
- shot
- process according
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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- Heat Treatment Of Steel (AREA)
- Turbine Rotor Nozzle Sealing (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
This is a process for producing improved chlor-ide corrosion resistance in turbine components fabricated from stainless steels which contains generally at least 1%
if both a martensitic and metastable austenitic phase and comprises an initial high-intensity shot peening, followed by an aging cycle at about 980-1020°F for ?-4 hours and a final (lower intensity) shot peening.
A relatively homogeneous surface of aged marten-site is produced and selective attack which forms sharp pit-like defects which initiate cracks is avoided.
This is a process for producing improved chlor-ide corrosion resistance in turbine components fabricated from stainless steels which contains generally at least 1%
if both a martensitic and metastable austenitic phase and comprises an initial high-intensity shot peening, followed by an aging cycle at about 980-1020°F for ?-4 hours and a final (lower intensity) shot peening.
A relatively homogeneous surface of aged marten-site is produced and selective attack which forms sharp pit-like defects which initiate cracks is avoided.
Description
25ii'7 49, 291 TMREE-STEP l`REATMENT OF STAINLESS STEELS
TO INCREASE RESISTANCE TO CHLORIDE CORROSION
BACKGROUND OF THE INVENTION
This invention relates to a technique for treat-ing stainless steel to reduce corrosion, and more particu-larly to stainless steels which con ain both a martensitic and metastable austenitic phase, such as 17-4 PH (AISI 630 modified) or AISI typ~ 301.
Before the development of this technique, chlor-ine induced corrosion assisted cracking was a very signifi-cant problem. In steam turbines, for example, chlorine ion environments had caused a large number of blade crack ing incidents resulting in significant turbine downtime.
SUMM~RY OF THE INVENTION
A three-step special surface treatment of stain-less steel blades which contain both martensitic and aus-tenitic phases has been developed to enhance corrosion re-sistance. The enhanced corrosion resistance is provided at the surface by first shot peening the surface at a high intensity to transform most of the austenite at the surface to untempered martensite; then a heat treatment is performed to convert the surface to largely all tempered martensitic; and finally the component surface is again shot peened, but at a lower intensity than in the initial step.
An embodiment or this process comprises an initial shot peening of the fabricated turbine component at an intensity of .010-.015A with 190-270 size shot. The shot peened com ponent is then heat-treated at 980-1020F for l/4-4 hours.
The heat-treated component is then given a final shot peening at an intensity of .004 .006A with 70-150 size shot.
.~ ~
TO INCREASE RESISTANCE TO CHLORIDE CORROSION
BACKGROUND OF THE INVENTION
This invention relates to a technique for treat-ing stainless steel to reduce corrosion, and more particu-larly to stainless steels which con ain both a martensitic and metastable austenitic phase, such as 17-4 PH (AISI 630 modified) or AISI typ~ 301.
Before the development of this technique, chlor-ine induced corrosion assisted cracking was a very signifi-cant problem. In steam turbines, for example, chlorine ion environments had caused a large number of blade crack ing incidents resulting in significant turbine downtime.
SUMM~RY OF THE INVENTION
A three-step special surface treatment of stain-less steel blades which contain both martensitic and aus-tenitic phases has been developed to enhance corrosion re-sistance. The enhanced corrosion resistance is provided at the surface by first shot peening the surface at a high intensity to transform most of the austenite at the surface to untempered martensite; then a heat treatment is performed to convert the surface to largely all tempered martensitic; and finally the component surface is again shot peened, but at a lower intensity than in the initial step.
An embodiment or this process comprises an initial shot peening of the fabricated turbine component at an intensity of .010-.015A with 190-270 size shot. The shot peened com ponent is then heat-treated at 980-1020F for l/4-4 hours.
The heat-treated component is then given a final shot peening at an intensity of .004 .006A with 70-150 size shot.
.~ ~
2~i7 la 49,291 BRIEF DESCRIPTION OF THE D~AWINGS
A better understanding of the invention may be had by reference to the drawings, in which:
6~
2 -49,291 Figure 1 is a graph indicating the relative per-cent of aged martensite, untempered martensite, and auste-nite as a function of depth after conventional treatment (shot peening at .004~.006A intensity);
Figure ~ is a similar graph of percent of the phases versus depth, but after the initial shot peening step of this invention;
Figure 3 is a similar graph of percent of the phases versus depth after the second (heating) step of this invention;
Figure 3A is a similar graph of percent of phases versus depth after the final shot peening step of this invention;
Figure 3B shows the percent austenite versus depth for a 17-4 ph stainless steel;
Figure 4A shows the fatigue strength of plain bar axial fatigue specimens tested in a chloride ion containing fatigue environment relative to fatigue strength in air; and Figure 4B shows notched fatigue properties of various specimens under the same chloride and air environ-ments.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
It has been found that stainless steel, which contains both martensitic and austenitic phases, have experienced pitting/cracking due to selective corrosion of the aged martensite (as opposed to equal attack on the three phases present in the microstructure's aged and unaged martensite, and austenite) when exposed to chloride ion bearing turbine environments. The selective nature of the attack results in increased sensitivity to cracX
initiation due to the sharp pit-like defects that are formed. Cracks propagate to such a depth that normal blade operating stresses, for example, can cause high cycle fatigue cracking and subsequent blade failure.
As can be seen from Figure 1, the normal shot peening at .004-006A intensity changes most of the auste-62~7
A better understanding of the invention may be had by reference to the drawings, in which:
6~
2 -49,291 Figure 1 is a graph indicating the relative per-cent of aged martensite, untempered martensite, and auste-nite as a function of depth after conventional treatment (shot peening at .004~.006A intensity);
Figure ~ is a similar graph of percent of the phases versus depth, but after the initial shot peening step of this invention;
Figure 3 is a similar graph of percent of the phases versus depth after the second (heating) step of this invention;
Figure 3A is a similar graph of percent of phases versus depth after the final shot peening step of this invention;
Figure 3B shows the percent austenite versus depth for a 17-4 ph stainless steel;
Figure 4A shows the fatigue strength of plain bar axial fatigue specimens tested in a chloride ion containing fatigue environment relative to fatigue strength in air; and Figure 4B shows notched fatigue properties of various specimens under the same chloride and air environ-ments.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
It has been found that stainless steel, which contains both martensitic and austenitic phases, have experienced pitting/cracking due to selective corrosion of the aged martensite (as opposed to equal attack on the three phases present in the microstructure's aged and unaged martensite, and austenite) when exposed to chloride ion bearing turbine environments. The selective nature of the attack results in increased sensitivity to cracX
initiation due to the sharp pit-like defects that are formed. Cracks propagate to such a depth that normal blade operating stresses, for example, can cause high cycle fatigue cracking and subsequent blade failure.
As can be seen from Figure 1, the normal shot peening at .004-006A intensity changes most of the auste-62~7
3 49,291 nite at or near the surface to untempered (unaged) marten~
site (which, like the austenite, is not as rapidly attacked as the aged martensite which, for example, con-stitutes about 75 percent of the material in a 17-4 PH
material). The untempered martensite in the surface layers is produced by strain transformation of austenite -to untempered martensite and varying the intensity o~ the shot peening will vary the depth of austenite transformed.
To avoid the selective chloride attack on por-tions o~ the surface and the sharp pit-like defects which initiate cracks, the surface is converted by the process of this invention to largely all tover 95%) tempered (aged) martensite such that the corrosion occurs rela-tively uniformly over the entire surface (avoiding phase selective pitting and cracking).
A three-step special surface treatment is used.
In the first step, the fabricated component (e.g. finished steam turbine blade) is high intensity shot peened all over (.010-.015A intensity, 230 shot size, 125-300% cover-age) to strain transform austenite (present in the micro-structure at typically 15-35%) to unaged martensite to produce a low amount of austenite in the surface layer (to a depth of 3-5 mils). The results of this shot peening are shown schematically in Figure 2.
In the second step, the component is heat-treat-ed (980-1020F for 1/4-4 hours) to age the material which had been transformed into unaged martensite (Figure 3) to cause primarily diffusion of nickel to the copper precipi-tate reducing the nickel in solution in the prior austen-ite and now aged martensite phase (austenite and untemp-ered martensite are primarily nickel rich relative to the aged martensite). The resultlng chemically homogeneous phases in the metal surface layer enhance corrosion re-sistance.
In the third step, the component is shot peened at lower intensity (.004~.006A to regain the compressive layer) which contributes to corrosion resistance and fatigue strength benefits (Figure 3A).
.~, ,~
3a 49,291 The measured percent of austenite in a 17-4 ph steel as a function of depth after each of the three stages of the present invention is shown in Figure 3B.
site (which, like the austenite, is not as rapidly attacked as the aged martensite which, for example, con-stitutes about 75 percent of the material in a 17-4 PH
material). The untempered martensite in the surface layers is produced by strain transformation of austenite -to untempered martensite and varying the intensity o~ the shot peening will vary the depth of austenite transformed.
To avoid the selective chloride attack on por-tions o~ the surface and the sharp pit-like defects which initiate cracks, the surface is converted by the process of this invention to largely all tover 95%) tempered (aged) martensite such that the corrosion occurs rela-tively uniformly over the entire surface (avoiding phase selective pitting and cracking).
A three-step special surface treatment is used.
In the first step, the fabricated component (e.g. finished steam turbine blade) is high intensity shot peened all over (.010-.015A intensity, 230 shot size, 125-300% cover-age) to strain transform austenite (present in the micro-structure at typically 15-35%) to unaged martensite to produce a low amount of austenite in the surface layer (to a depth of 3-5 mils). The results of this shot peening are shown schematically in Figure 2.
In the second step, the component is heat-treat-ed (980-1020F for 1/4-4 hours) to age the material which had been transformed into unaged martensite (Figure 3) to cause primarily diffusion of nickel to the copper precipi-tate reducing the nickel in solution in the prior austen-ite and now aged martensite phase (austenite and untemp-ered martensite are primarily nickel rich relative to the aged martensite). The resultlng chemically homogeneous phases in the metal surface layer enhance corrosion re-sistance.
In the third step, the component is shot peened at lower intensity (.004~.006A to regain the compressive layer) which contributes to corrosion resistance and fatigue strength benefits (Figure 3A).
.~, ,~
3a 49,291 The measured percent of austenite in a 17-4 ph steel as a function of depth after each of the three stages of the present invention is shown in Figure 3B.
4 49,291 Test specimens were exposed to a chloride ion containing fatigue testing environment (24% sodium chlor-ide, ~4.5% Na2S04 solution boiling at 225F, deaerated to 20 ~P~-O2 at a pH of 7.5-8.5) to evaluate the results. As indicated in Figure 4A, the treatment of this invention ~5~, provides a more than two times increase in atigue strength when compared to the conventional treatment (both sets of specimens tested in the aforementioned fatigue environment). As can be seen from Figure 4B, the notched fatigue properties of the treated material in the environ-ment is equivalent to the standard material properties in the unnotched condition.
As can be seen from Table I below, test results for slow strain rate testing also suggests some improve-ment.
6~
49,291 TABLE I
__ SLOW STRAIN RATE TEST SUM~IARY
SPECI~IEN PREPARATION ENV. UTS Y.S. EL R.A.
(KSI) (KSI) (%) (~0) BX-SP (BASE-LINE) AIR,R.T. 142 99 l5.6 69.9 "A" 115103 6.0 19.6 ~, SP1 .010-.015A,170SS "A"115 112 8 43.0 1000F, 1/2 HR.
SPF- .004-.006A, 110SS
SAME AS ABOVE, 1000F, 20 HRS. "A" 123120 10 30.1 -SP1 .010-.015A,230SS "A"124 11~ 11 34.0 1000F, 1/2 HR.
As can be seen from Table I below, test results for slow strain rate testing also suggests some improve-ment.
6~
49,291 TABLE I
__ SLOW STRAIN RATE TEST SUM~IARY
SPECI~IEN PREPARATION ENV. UTS Y.S. EL R.A.
(KSI) (KSI) (%) (~0) BX-SP (BASE-LINE) AIR,R.T. 142 99 l5.6 69.9 "A" 115103 6.0 19.6 ~, SP1 .010-.015A,170SS "A"115 112 8 43.0 1000F, 1/2 HR.
SPF- .004-.006A, 110SS
SAME AS ABOVE, 1000F, 20 HRS. "A" 123120 10 30.1 -SP1 .010-.015A,230SS "A"124 11~ 11 34.0 1000F, 1/2 HR.
5~
~R~ .004-.006A, 110SS
SAME AS ABOVE, 1000F, 20 HRS. "A" 127123 8 37.0 ENV.A-6 gms Na2S04 + 33 gms NaC1 + Oxide Mixture "K", boiling (220F) 2 20ppb SPF - FINAL SHOT PEENING
The initial high intensity shot peening is done at an intensity of .010-.015A (and preferably .010-.012A) with a shot size of 190-270 (preferably 210-250 and typi-cally 230~ and preferably with about 150~ coverage. The heating is preferably done to 990-1000F for about ~ to 2
~R~ .004-.006A, 110SS
SAME AS ABOVE, 1000F, 20 HRS. "A" 127123 8 37.0 ENV.A-6 gms Na2S04 + 33 gms NaC1 + Oxide Mixture "K", boiling (220F) 2 20ppb SPF - FINAL SHOT PEENING
The initial high intensity shot peening is done at an intensity of .010-.015A (and preferably .010-.012A) with a shot size of 190-270 (preferably 210-250 and typi-cally 230~ and preferably with about 150~ coverage. The heating is preferably done to 990-1000F for about ~ to 2
6 49,291 hours. The final shot peening is done at an intensity of .004 .006A with 70-150 shot size (preferably 90-130 and typically 110 shot size) and preferably with 150% cover-age).
Claims (12)
1. A process for producing improved chloride corrosion resistance in turbine components fabricated from stainless steel which contains both aged martensitic and metastable austenitic phases, said process comprising:
(a) initial shot peening said fabricated component at an intensity of .010-.015A with 190-270 size shot;
(b) heating said component to about 980-1020°F for 1/4 - 4 hours; and (c) final shot peening said component at an intensity of .004- .006A with 70-150 size shot.
(a) initial shot peening said fabricated component at an intensity of .010-.015A with 190-270 size shot;
(b) heating said component to about 980-1020°F for 1/4 - 4 hours; and (c) final shot peening said component at an intensity of .004- .006A with 70-150 size shot.
2. The process of claim 1, wherein said compo-nent is heated to 990-1000°F for 1/2 to 2 hours.
3. The process of claim 2, wherein 125-300%
coverage is provided during both shot peenings.
coverage is provided during both shot peenings.
4. The process of claim 3, wherein the initial shot peening is at an .010- .012A intensity with 210-250 size shot, with about 150% coverage.
5. The process of claim 4, wherein the final shot peening is with 90 130 size shot, with about 150% coverage.
6. A process for producing improved chloride corrosion resistance in turbine components fabricated from stainless steel which has a surface containing both aged martensitic and metastable austenitic phases, said process comprising:
(a) initial high-intensity shot peening of said surface of said component transforming most of said metastable austenite at said surface to martensite;
(b) then heat treating to convert said surface to largely all aged martensite;
(c) and then shot peening said surface at a lower intensity than in said initial high intensity shot peening step.
(a) initial high-intensity shot peening of said surface of said component transforming most of said metastable austenite at said surface to martensite;
(b) then heat treating to convert said surface to largely all aged martensite;
(c) and then shot peening said surface at a lower intensity than in said initial high intensity shot peening step.
7. The process according to claim 1 wherein said stainless steel further contains precipitates formed by aging.
8. The process according to claim 1 wherein said stainless steel is a 17-4 PH stainless steel.
9. The process according to claim 7 wherein said precipitates contain copper.
10. The process according to claim 6 wherein said stainless steel surface further contains precipitates formed by aging; and after the final shot peening step said stainless steel surface is characterized by said aged martensitic phase, and precipitates containing copper.
11. The process according to claim 10 wherein said stainless steel is 17-4 PH stainless.
12. The process according to claim 10 wherein after said final shot peening step said stainless steel surface is further characterized by a low amount of austenitic phase.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US26782681A | 1981-05-27 | 1981-05-27 | |
| US267,826 | 1981-05-27 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1196257A true CA1196257A (en) | 1985-11-05 |
Family
ID=23020285
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000402682A Expired CA1196257A (en) | 1981-05-27 | 1982-05-11 | Three-step treatment of stainless steels having metastable austenitic and martensitic phases to increase resistance to chloride corrosion |
Country Status (2)
| Country | Link |
|---|---|
| JP (1) | JPS57200519A (en) |
| CA (1) | CA1196257A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN116623101A (en) * | 2023-07-21 | 2023-08-22 | 北京中科万德创新科技有限公司 | Cu-containing antibacterial stainless steel material with high hardness and corrosion resistance, and preparation method and application thereof |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPH0699739B2 (en) * | 1986-04-25 | 1994-12-07 | 石川島播磨重工業株式会社 | Surface hardening method for cast iron parts |
-
1982
- 1982-05-11 CA CA000402682A patent/CA1196257A/en not_active Expired
- 1982-05-27 JP JP8896182A patent/JPS57200519A/en active Granted
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN116623101A (en) * | 2023-07-21 | 2023-08-22 | 北京中科万德创新科技有限公司 | Cu-containing antibacterial stainless steel material with high hardness and corrosion resistance, and preparation method and application thereof |
| CN116623101B (en) * | 2023-07-21 | 2023-09-19 | 北京中科万德创新科技有限公司 | Cu-containing antibacterial stainless steel material with high hardness and corrosion resistance, and preparation method and application thereof |
Also Published As
| Publication number | Publication date |
|---|---|
| JPS57200519A (en) | 1982-12-08 |
| JPH021892B2 (en) | 1990-01-16 |
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