SAE ARP 982D-2007 Minimizing Stress-Corrosion Cracking in Wrought Titanium Alloy Products《钛合金制品中断裂的最小化金属超应力引起的腐蚀》.pdf

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4、0 (outside USA) Fax: 724-776-0790 Email: CustomerServicesae.org SAE WEB ADDRESS: http:/www.sae.org SAE values your input. To provide feedback on this Technical Report, please visit http:/www.sae.org/technical/standards/ARP982D AEROSPACE RECOMMENDED PRACTICE ARP982 REV. D Issued 1967-11 Revised 2007-

5、12Reaffirmed 2016-09 Superseding ARP982C Minimizing Stress-Corrosion Cracking in Wrought Titanium Alloy Products RATIONALE ARP982D has been reaffirmed to comply with the SAE five-year review policy. 1. SCOPE Primarily to provide recommendations concerning minimizing stress-corrosion cracking in wrou

6、ght titanium alloy products. 1.1 The detailed recommendations are based on laboratory experience and reflect those design practices and fabrication procedures which should obviate in-service stress-corrosion cracking of wrought titanium alloy products. 1.2 It must be emphasized that while stress-cor

7、rosion cracking in service has been observed, the chemical environmental conditions have, in all instances, been unusual and, although it is possible to produce stress-corrosion cracking of titanium alloys under more common conditions as discussed in these recommendations, there have been few, if an

8、y, failures in such environments. 2. APPLICABLE DOCUMENTS The following publications form a part of this document to the extent specified herein. The latest issue of SAE publications shall apply. The applicable issue of other publications shall be the issue in effect on the date of the purchase orde

9、r. In the event of conflict between the text of this document and references cited herein, the text of this document takes precedence. Nothing in this document, however, supersedes applicable laws and regulations unless a specific exemption has been obtained. 2.1 SAE Publications Available from SAE

10、International, 400 Commonwealth Drive, Warrendale, PA 15096-0001, Tel: 877-606-7323 (inside USA and Canada) or 724-776-4970 (outside USA), www.sae.org. ARP1795 Stress Corrosion of Titanium Alloys, Effect of Cleaning Agents on Aircraft Engine Materials SAE INTERNATIONAL ARP982D Page 2 of 4 3. GENERAL

11、 All metal alloy systems are subject to stress-corrosion cracking under appropriate conditions; titanium alloys are no exception to this rule. Stress-corrosion failures of wrought titanium alloy parts are possible if the following combination of factors is met: a. Presence of a sustained high surfac

12、e tensile stress developed as a result of assembly stresses or residual stresses due to heat treatment or forming (or plane strain produced by the tensile stress concentration at the root of a pre-existing crack), and b. Presence of environmental conditions (media, temperature, stress, time) specifi

13、c to the material under consideration. 3.1 There are several types of environments in which stress cracking of titanium alloy parts may occur. These environments and the alloys currently known to be susceptible to cracking in each type are: a. Environments in which both initiation and propagation ma

14、y occur at low stress concentration factors; that is, cracking of smooth machined components may occur at sufficiently high applied stress. 1. Alkali halide salts (and halide salts of some other metal) above 500 F (260 C) (oxygen and water vapor are contributing factors). In general, all commerciall

15、y available titanium alloys are believed to be susceptible to hot salt cracking. 2. Silver, cadmium, and mercury have been found to promote cracking in titanium alloys; temperature (even room) and stress limits for cracking have not been firmly established and caution should therefore be exercised.

16、3. Methyl alcohol (methanol). Susceptibility of titanium to SCC in methanol is the greatest at room temperature 68 F (20 C) with increasing temperatures resulting in decreased SCC susceptibility. Time to cracking is decreased by the presence of halides, acidification, or presence of strong oxidizers

17、. A minimum water content of 3 wt. % is sufficient to prevent SCC for most titanium alloys during sustained exposure to methanol solutions. 4. Nitrogen tetroxide (N2O4). Titanium 6Al-4V tankage holding N2O4with no measurable amounts of NO in a temperature range of 86 to 167 F (30 to 75 C) has failed

18、 by stress corrosion. Other titanium alloys are also susceptible. Cracking is inhibited by presence of 0.4 to 0.8 % NO in propellant grade N2O4. 5. Red fuming nitric acid caused one of the early stress-corrosion problems encountered with titanium. It has been determined that red fuming nitric acid w

19、ith less than 1.5 % water and less than 10 to 20 % NO2will aggressively attack the metal and result in a pyrophoric reaction. b. Environments in which brittle cracking may occur under high stress concentrations; that is, a pre-existing crack may propagate under applied stress, provided component geo

20、metry is such as to produce plane strain conditions. All titanium alloys are believed to be susceptible in some degree, with the exception of low-oxygen, low-iron titanium such as the commercially pure alloys. The following environments may produce this susceptibility: 1. Aqueous solutions of alkali

21、 halides; the lower limit of halide salt concentration has not been firmly established and caution, even in pure water, should be exercised. 2. Many organic liquids, notably anhydrous methanol and halogenated hydrocarbons, also produce susceptibility to some degree. 3. Mercury and cadmium. SAE INTER

22、NATIONAL ARP982D Page 3 of 4 3.2 Performance Under Service Conditions Severe stress-corrosion cracking of titanium alloys has been encountered in highly-stressed pressure vessels containing methyl alcohol or containing N2O4free of NO. Also, small cracks have been seen in wrought titanium components

23、exposed to halide salts during heat treatment. With these exceptions, stress-corrosion cracking has rarely, if ever, occurred under service conditions. The alloys Ti-6Al-4V ELI and Ti-6Al-2Sn-4Zr-2Mo in particular exhibit low susceptibility. While laboratory testing showing the possibility of stress

24、-corrosion cracking indicates a need for caution, service experience with titanium alloys has been extremely good compared with other structural materials. 4. RECOMMENDATIONS 4.1 Environmental Compatibility Alloys should be selected which offer adequate resistance to stress-corrosion for the environ

25、ment in which they are to be used. Alloys with intermediate resistance are Ti-6Al-4V, Ti-6Al-6V-2Sn, and Ti-13V-11Cr-3Al. Although not commercially produced anymore, Ti-4Al-3Mo-1V is one of the most resistant alloys. Alloys that are highly resistant to seawater crack propagation under plane strain c

26、onditions include Ti-4Al-3Mo-1V, Ti-6Al-2Cb-1Ta-1Mo, and Ti-6Al-4V ELI. 4.1.1 Addition of small amounts of water to methyl alcohol and either water or nitrogen oxide (NO) to N2O4tends to inhibit stress-corrosion by these environments. 4.2 Processing and Heat Treatments Studies have indicated that he

27、at treatment and processing variables play a major role in sensitizing titanium alloys to stress-corrosion cracking. Beta processing followed by normal heat treatments, or normal processing plus beta heat treatments reduces or eliminates sensitivity to stress-corrosion cracking of most alloys in aqu

28、eous environments. However, beta structures often have lower tensile properties with reduced ductility. The effectiveness of these treatments varies with the alloy grades and does not necessarily apply to other forms of stress-corrosion. For example, beta processing or heat treatment generally incre

29、ases the sensitivity of alloys to hot salt corrosion while imparting resistance to room temperature aqueous stress-corrosion cracking. 4.3 Design Stresses Threshold stresses that develop hot salt stress-corrosion cracking are often determined in laboratory testing of simple specimens with salt coati

30、ngs; other conditions not necessarily simulating operations may be used as a guide in limiting maximum tensile stresses. However, experience has shown, at least in the instance of aircraft turbine engines, that these laboratory derived stresses are conservative when compared with stresses found to b

31、e safe in service. There is no background of experience relating to the propagation of brittle cracks under the influence of water or saline solutions but, on general principles, high stress concentration factors should be avoided. Depending on circumstances, this may demand that the fatigue enduran

32、ce limit be used as the design limit for cyclic stresses. When high sustained loads at high stress concentration factors are unavoidable, an alloy with low susceptibility to propagation of brittle cracks in the presence of aqueous solutions should be used. Such alloys include Ti-6Al-2Cb-1Ta-1Mo and

33、Ti-6Al-4V-ELI. 4.4 Surface Compressive Stresses The above-mentioned design limitations are generally raised when surface compressive stresses are introduced, such as by shot peening. 4.5 Protective Coatings Research has shown that continuous nonporous coatings can prevent hot salt stress-corrosion c

34、racking. However the adverse effects on engineering properties and the problems of maintaining a continuous nonporous coating discourage any commercial usage. SAE INTERNATIONAL ARP982D Page 4 of 4 4.6 Cleaning Solutions Deionized water and nonhalogenated organic solvents (such as methyl ethyl ketone

35、 and acetone), with the exception of methyl alcohol, can safely be used for cleaning. Chlorinated hydrocarbons or the residues of chlorinated hydrocarbons can cause cracking of certain titanium alloys at elevated temperatures. Although the incidence of such cracking is low, precautions should be tak

36、en to remove residues prior to heat treatment. Titanium alloy components should not be exposed to high temperature vapors of halogenated organic fluids. 4.7 Stress-Corrosion Testing of Turbine Engine Cleaners and Maintenance Chemicals ARP1795 describes stress-corrosion testing methods recommended fo

37、r evaluating the effect of turbine engine cleaners and maintenance materials on titanium alloys. 5. NOTES 5.1 The change bar ( l ) located in the left margin is for the convenience of the user in locating areas where technical revisions, not editorial changes, have been made to the previous issue of this document. An (R) symbol to the left of the document title indicates a complete revision of the document. PREPARED BY AMS COMMITTEE “G“