Introduction
Stainless steel is an alloy with iron as major part and chromium with at least 11% by weight . Stainless steel industry grew rapidly during the 1989-93 period . This paper analyzes stress corrosion of stainless steel along with prevention measures and selection methods for stainless steel. After carrying out the study, it would be established whether stainless steel is an effective alloy or not for major uses in steel industry through adopting various selection parameters. Stainless steel is called stainless due to the resistance to rusting and corrosion .
Stress-corrosion in Stainless Steel
In order to reduce various occurrences of intergranular stress-corrosion cracking and severe sensitization in the components of stainless steel, complete control on processing and application of stainless steel is needed. It has been demonstrated by the test data that stainless steel which is sensitized is more susceptible to the intergranular stress-corrosion cracking as compared to non-sensitized stainless steel which is a heat treated solution. In order to carry out carburization of austenitic stainless steel through using liquid sodium, the reaction rates in liquid sodium system was found to be quite rapid in a study .
The austenitic unsterilized stainless steels are of more specific concern which is composed by American Steel and Iron Institute. It is recommended that exercising of process control procedures should be as per knowledge and practices being done in manufacturing and welding processes. The knowledge and operating experiences are gained during manufacturing and construction of reactors to reduce the exposure of steel to the contaminants. This exposure could lead the stainless steel to corrosion of stress cracking.
Testing and Prevention Measures
The steel products produced through alloy forming have a different level of various elements including carbon as compared to stainless steel products and stainless steel provides higher level of corrosion resistance as compared to alloy tool steel . All the cleaning solutions, degreasing agents, processing compounds and other material should completely be removed from any processing stage prior to any hydro test and temperature treatment as per the guidelines of the approved elevated / manufacturing temperature treatment process. In this context, a number of factors are to be kept in mind and precaution should be undertaken. The area of construction and fabrication should be kept clean. The components should be protected and kept dry during the process of shipment and storage. All the tiny openings and crevices should be protected against the contamination as notified in the procedures mentioned in quality assurance manual.
Procedures that are based on the modification of Conventional Strain method on the Partitioning method have been proposed in a study while characterizing degradation of alloys at high temperature and low-cycle fatigue based on time . Sensitization of stainless steel increases with the decreasing rate of cooling and appears to be decreasing with the increase of heating rate . Various alloys of iron have a number of applications based on the composition of the alloy. There is a vast field of study on the applications of alloys. Some alloys are even used in nuclear reactors based on their strength and high-temperature resistivity .
Delta ferrite content in stainless steel results in developing fatigue cracks growth resistance . Avoidance of pickling of the sensitized stainless is strongly recommended. Similarly, surface contamination should also be avoided by taking special precautions by welding with fluxes and rod coating fluorides. It is very significant that final washing of the finished surface should be carried out by extremely high-quality water. In this pretext, regulatory guide No 1.37 would be of great use.
The solution of heat testing and treating should normally be done on the starting of material. However, in order to ensure good conditions for the solution of heat-treatment of finished components’ surface area, it is suggested that the heat testing and treating operation may be performed at the later stages of manufacturing of component. The process of heat treating of the solution should be comprised of a rapid rate of cooling in order to avoid precipitation to the carbides so that intergranular stress-corrosion may not happen.
A good cooling rate may be produced by water quenching. However, the cooling done by any other means other than quenching of water in only acceptable when the rate of cooling is significantly rapid in order to avoid sensitization. Some of the carbide precipitation result during the process of welding of weld metal and in the heat affected zone of base metal. This sensitization, however, does not occur when material chemistry and welding procedures are used. It does not occur when the material is not further heated. It is evident from the experience that if high input heat is used in welding, it could result in corrosion cracking. In order to prevent this action, material chemistry and welding procedures are supposed to be controlled so that sensitization of zones of affected heat may be prevented.
Few suggestions which are widely used to prevent excessive sensitization can be considered. The welding practices should be avoided which result in the production of the high rate of heat. By controlling voltage, current and speed of travel, the input of low heat can be maintained. The inter-pass temperature is limited. The bead stringer techniques are used and excessive weaving is avoided. The material’s carbon level be made limited where the thickness of section makes material exposed for sensitization .
Although at this time, no standard test does exist to determine the susceptibility of cracking of underclad, however, several methods of the test have been developed for the same purpose and have also been considered as satisfactory to determine the cracks existence. One of the tests of this kind is comprised of a method in which the cladding is removed from the line of fusion. It also includes base material’s examination for cracks, and in this regards following methods for examinations are used.
Liquid penetrant, metallography or the magnetic particle. This is done after progressive examination and grinding through Heat Affected Zone (HAZ) until all the cracks are revealed completely. Another method of test is used in which three specimens’ cross sections are to make either parallel or transverse to examine the HAZ, cracks base metal, weld and weld examination. The exercise of controlling should be carried out in order to limit underclad cracking occurrence in the components of steel of low alloy material and components related to safety with clad stainless steel. It is recommended that the process of welding which induces cracking after extra promoting and heating in metal’s base should be avoided to use for the purpose of cladding with known cracking susceptibility .
According to a study, the hardness of welded stainless steel is much higher than most of the other metals and alloys. This shows that stainless steel can be welded with better results in parts where welding is necessary as compared to other metals or alloys. There is more than 10 percent of chromium in the stainless steels which are based on iron alloys. These have been used for most of the architectural, chemical industrial, and applications of customers for the last many years. A number of kinds of steels have been marketed in the recent markets duly recognized by AISI (American Iron & Steel Institute). Few of the steels are available commercially and are termed as stainless proprietary steels having specific characteristics. As there are a number of types or steels are present in the market, it is difficult to choose the required and best one. For this purpose, there should be a source of information depicting the capabilities and characteristics of useful alloys.
For the purpose of guidance, a booklet has been prepared after detailed consensus by a committee responsible for the production of stainless steel. The data has been updated and reviewed by SSINA (Special Steel Industry of the North America). A booklet has specially been written by design engineers. An overview has been presented in the booklet regarding all ranges of stainless steel, their proprietary, and standard, composition, properties, fabrication and uses.
Engineers of AIME (American Institution of Metallurgical, Mining, and Petroleum engineers) have presented excessive information on standards grades of stainless steel with extra characteristic and features. The publication of a book by AIME took place at the time when the publication of AISI manual was discontinued since 1986. Generally, there are more than 100 alloys treated as stainless steel. However, there are three major classifications which are being used for identification of stainless steel. First is Metallurgical structure, second includes the number of systems has been produced by ISI; the series numbers are 200, 300 & 400 and lastly the system of unified numbering; this system was established by ASTM (American Society Testing Materials) and SAE (Society of the Automotive Engineer).
There are few grades in the market and have the same nomenclature as given by AISI designations. ASTM has also recognized these names. These common nomenclatures are neither associated with one producer nor a trademark, are identified and shown in the tables. These names are also mentioned in the specification of ASTM. It is recognized that all types of steels have designations of UNS which are being used in areas of North America.
Guidelines for Steel Selection
It has been concluded in a study that there is no single type of stainless steel that can be effectively selected for high performance in all conditions . However, the stainless steels have a good resistance to corrosion, fabrication and strong characteristics and are considered the best engineering materials. They readily can meet an excessive range of a designed criteria, service life, load and low maintenance. If proper kind of stainless steel is selected means following important factors will be observed in future:-
1. Corrosion / Heat Resistance
The main purpose is to specify the type of stainless steel. The corrosion degree and nature of environment or resistance of heat needed should be known by the recommender. The resistance to heat / corrosion is an important and critical parameter that is to be considered while selecting the type of steel for any particular application. Stainless steel is highly resistant to corrosion and heat due to the Chromium Oxide layer formed on the surface of the steel.
2. Mechanical Properties
With specific emphasis on the room strength, low-temperature elevation should be known. The combination of strength and corrosion resistance is a main condition of selection. Mechanical properties play another critical role in the selection of the type of steel due to the fact that the primary purpose of using steel based material is to get mechanical strength and other features that are the result of mechanical properties.
3. Fabrication Operations
The consideration at third level is to know that how steel is to be finalized. This operation comprised of machining, forging, forming and welding. The type of fabrication processes affects the cost of steel production as well as the type of steel formed. More sophisticated fabrication results in lesser impurities and structural defects in the fabricated material.
4. Total Cost
Total cost does not mean the cost incurred in production and purchasing the related material, rather the cost of life cycle comprising benefits of cost saving for maintenance is also included. The total cost of producing a specific type of steel is a major economic / financial factor for using a type of steel in a specific application. The total cost of steel production increases with the better type of steel and decreases with better production processes / tools.
Corrosion Resistance
Chromium is an element which is responsible for resistance of steel to avoid corrosion. This material has qualities against the corrosion action by combination with O2 and forms an invisible and thin protective film on the surface of the steel. The disruption of such film is to be avoided by using the proper material. If the protective film is destroyed or disturbed, it will be reformed in the presence of O2 and persistently give protection. The film improved the life of the material from being destroyed.
Material Selection
The corrosive environment has been characterized by a number of variables; these are atmospheric conditions, a chemical with its concentration, time, and temperature. Therefore, it is very cumbersome to decide the selection of alloy without exact knowledge of the environment and its nature .
The material used to produce stainless steel was considered as an expensive material. It is 15 times expensive as compared to the ordinary steel. A metallurgist of young age in the year 1970 discovered a procedure which cut the cost of the stainless steel into about half. This process produced better steel as it was available at that time. This story was told by metallurgist by himself and the second part of the story is that to discover the process took almost 12 years that how the better quality of steel could be produced at the large scale .
Stainless steel in part can be produced for heat resistance applications through replacing part of Chromium with Silicon or Aluminum as proved in a study . Traditionally, a number of procedures are adapted to sort out the scrap metals. The sorting and identification of the scrap metals are being carried out by the workers who are skilled and experienced in this field. Various chemical and physical tests are used for sorting of the scrap material. New technologies for identification of materials are used; for instance, emission spectroscopy, thermoelectric response, and spectrograph of fluorescent X-rays. These techniques have tremendously improved the ease and accuracy of instant identification.
In this context, about 27 samples of super alloy and stainless steel were used for investigations in the lab by these technologies. The determination of the method of identification to be used depends upon offered potential value by the degree of the separation. However, use of any mentioned technique may not ensure identification of lesser or higher valuable materials. Moreover, some of the additional amounts of presorting are mandatory. For the purpose of identification, it should be known that what sort of material is present in the scrap so that calibration instruments used for identification may be chosen. Thermoelectric response inherently is considered as an identification method for comparison as the elements individually cannot be determined quantitatively.
It is although fundamentally capable of the analysis of material quantitatively, the emission spectroscopy also generally is utilized for a method of comparison. The response of the process is a number of elements individually, however, it is quite difficult to know the patterns of specific alloy or to determine accurately the quantitative amounts until and less manpower with considerable skill and training is available. The needed skills for determining the alloy types and patterns can easily be learned. The calculating presence of the amount of the element present in various sample is also very easy to learn. The optical spectroscopy for emission and thermoelectric sorter have been mixed in order to form a low-cost system of sorting duly effective and efficient in its working. Many alloys have been identified by X-ray spectrograph techniques.
It has been observed from the data of sorting of material, that sometimes misidentification of an alloy took place due to the reason of less programmed identifying elements. However, it is also revealed that the results of these actions are excellent when the instrument of specified class is utilized within the probable range of the alloys. As compared to knowing of an alloy directly through identifying its name, the identification of alloy through certain procedures is a cumbersome and tedious process. For unknown material, several minutes are required to identify within the range of design. For comparing the outcome of the result, elemental analysis is used and the specification of the alloy is also to compare with the results. This analysis requires significant time for identification of alloy .
Comparison of the protective properties can be made when stainless steel is exposed to a temperature in a critical range. This crucial range for temperature differs for various types of stainless steels. The temperature is a bit higher as compared to the temperatures used for other materials. After putting the stainless steel at a prescribed temperature for specific time and degrees, makes the material heat sensitized. The term heat sensitization means that protective elements of stainless steel decreased to levels where the corrosion do not occur and it protects the material from becoming corroded.
The sensitization depends upon the time and temperature before sensitization of heat occurs. For instance, the material which is to be made heat sensitized may be needed to expose to the temperature for about a year or may more than that before the occurrence of heat sensitization. This time of 01 years for making heat sensitization occurs when the temperature is used at the extreme lower end. On the other hand, at the upper end of the scale of temperature, the heat sensitization could be done within seconds. This action of higher temperature other than temperatures may be carried out in various ways.
These ways are comprised of storage or shipping of container, loading of extra sources, managing the sources in proper configuration within storage or shipping container and utilization or accidental conditions which cause in experiencing material in high temperatures. If the sensitized material is placed in a situation which is quite favorable for corrosion, the process of corrosion can be initiated provided that liquid medium for inter-granular corrosion is present. The rate of corrosion, in this case, is hard to be predicted and is typically dependent upon a number of factors. The main factors in this pretext are environment, temperature, and time.
It is strongly recommended by the management that all the licensees who are producing the stainless steel are supposed to be well aware of sensitization and potential factors which are the main causes of corrosion in Stainless steel. These are also supposed to take appropriate actions accordingly and should create own circumstances. Licensees particularly should be very sensitive to all the potential and past circumstances involved in transportation, incident responses, and accident conditions.
The licensees should take care of exposing their products in circumstances where sensitization could occur. They should avoid making their devices or material exposed to such sensitization. It should be monitored by the licensees that which of the devices or sources are suspected to the circumstance for being sensitized. They should also monitor the devices which have signs of the corrosion. If such signs are found in some of the devices, it is recommended that these should be separated from the others and should be placed in a condition which prevents corrosion. Before usage of these devices, careful evaluation should be carried out. In this context, knowledgeable consultants or manufacturers may also be consulted . Control of the processing and application of stainless steel can be carried out for avoiding severe sensitization is required to reduce numerous amounts of stress-corrosion of intergranular type cracking .
Conclusion
Stress corrosion in steel can be prevented through effective selection methods as well as prevention measures. It has been established that stainless steel is an effective alloy for major uses in steel industry through adopting various selection parameter. Stainless steel has a number of uses with two major purposes including good shining surface and resistance against rusting / corrosion. We need to monitor the devices which have signs of the corrosion. If any of these signs is found in some of the devices, it is recommended that these should be separated from the others and should be placed in a condition which prevents corrosion. Before usage of these devices, careful evaluation should be carried out.
References
American Iron and Steel Institute. Committee of Stainless Steel Producers. (1977). Design guidelines for the selection and use of stainless steel. (Designers' handbook series). Washington D.C.: American Iron and Steel Institute.
Anderson, W., & Sneesby, G. (1960). Carburization of Austenitic Stainless Steel in Liquid Sodium. (AEC research and development report). Canoga Park, California: Atomics International. Div. of North American Aviation, Inc.
Atteridge, D., Cedeno, C., & Engineering, U. N. (1991). Continuous cooling thermal cycle effects on sensitization in stainless steel (NUREG/GR ; 0002). Washington, DC: Division of Engineering, Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission.
Brown, R. D., Riley, J. W., & Zieba, C. A. (1984). Rapid Identification of Stainless Steel and Superalloy Scrap (Report of investigations (United States. Bureau of Mines) ; 8858). United States Dept. of the Interior, Bureau of Mines.
Cobb, H. M. (2010). The History of Stainless Steel. Materials Park, OH, USA: ASM International.
Davis, J. R., & Committee, A. I. (1994). Stainless Steels (ASM specialty handbook). Materials Park, Ohio: ASM International. Handbook Committee.
Fulcher, N. T., & Industries, U. S. (1995). Stainless steel mill products. (Industry & trade summary). Washington, DC: Office of Industries, U.S. International Trade Commission.
Gessel, G. R. (1978). Effects of minor alloying additions on the strength and swelling behavior of an austenitic stainless steel. Ames, Iowa: Iowa State University of Science and Technology.
Hawthorne, J. R., & Laboratory, N. R. (1978). Significance of Delta Ferrite Content to fatigue crack growth resistance of austenitic stainless steel weld deposits (NRL report ; 8201). Washington: [Dept. of Defense, Naval Dept., Office of Naval Research], Naval Research Laboratory.
Kalluri, S., Manson, S. S., Halford, G. R., & Administration, U. S. (1987). Environmental Degradation of 316 Stainless Steel in High Temperature Low Cycle Fatigue (NASA technical memorandum ; 89931). Washington, DC: [Springfield, Va.]: National Aeronautics and Space Administration ; [For sale by the National Technical Information Service].
Lai, J. K., Shek, C. H., & Lo, K. H. (2012). Stainless Steels: An Introduction and Their Recent Developments. Sharjah: Bentham Science Publishers.
Larson, M. L. (1984). Reduced-Chromium Stainless Steel Substitutes Containing Silicon and Aluminum (Report of investigations (United States. Bureau of Mines) ; 8918). Pittsburgh, Pa: U.S. Dept. of the Interior, Bureau of Mines.
Segan, E. G., Bukowski, J., Uyeda, H., Kumar, A., & Laboratory., C. E. (1982). Wrought Stainless Steel Fasteners for Civil Works Applications (Technical report (Construction Engineering Research Laboratory) ; M-306). Champaign, Ill [Springfield, Va.]: Construction Engineering Research Laboratory, U.S. Army, Corps of Engineers ; [National Technical Information Service, distributor].
U.S. Nuclear Regulatory Commission. (2011). Control of the processing and use of stainless steel. U.S. Nuclear Regulatory Commission.
U.S. Nuclear Regulatory Commission. Office of Nuclear Material Safety Safeguards. (1996). Vulnerability of Stainless Steel to Corrosion when Sensitized (NRC information notice ; 96-54). Washington, D.C.: U.S. Nuclear Regulatory Commission, Office of Nuclear Material Safety and Safeguards.
U.S. Nuclear Regulatory Commission. Office of Nuclear Regulatory Research, author, issuing body. (2011). Control of Stainless Steel Weld Cladding of Low-alloy Steel Components (Revision 1. ed., Regulatory guide (U.S. Nuclear Regulatory Commission. Office of Nuclear Regulatory Research) ; 1.43). Washington, DC: U.S. Nuclear Regulatory Commission, Office of Nuclear Regulatory Research.
U.S. Nuclear Regulatory Commission. Office of Nuclear Regulatory Research, author, issuing body. (2011). Control of the processing and use of stainless steel (Revision 1. ed., Regulatory guide (U.S. Nuclear Regulatory Commission. Office of Nuclear Regulatory Research) ; 1.44). Washington, DC: U.S. Nuclear Regulatory Commission, Office of Nuclear Regulatory Research.
United States International Trade Commission. (1983). Stainless steel and alloy tool steel : Report to the President on investigation no. TA-201-48 under section 201 of the Trade act of 1974. (USITC publication ; 1377). Washington, D.C.: U.S. International Trade Commission.
