Austenitic stainless steels cracking

Considering the success of detecting crack tip echoes from defects at the near probe surface, future work will deal with the detection and sizing of defects on the far probe surface. Future work also relates to carrying out defect sizing in anisotropic austenitic stainless steel welds and... 

Virtuallv evety alloy system has its specific environment conditions which will prodiice stress-corrosion cracking, and the time of exposure required to produce failure will vary from minutes to years. Typical examples include cracking of cold-formed brass in ammonia environments, cracking of austenitic stainless steels in the presence of chlorides, cracking of Monel in hydrofluosihcic acid, and caustic embrittlement cracking of steel in caustic solutions. 

A specific corrodent. One of the unusual and interesting features of SCC is the specificity of the corrodent. A particular alloy system is susceptible to SCC only when exposed to certain corrodents, some or all of which may be unique to that particular alloy system. For example, austenitic stainless steels (300 series) are susceptible to cracking in chloride solutions but are unaffected by ammonia. Brasses, on the other hand, will crack in ammonia but remain unaffected by chlorides. The corrodent need not be present at high concentrations. Cracking has occurred at corrodent levels measured in parts per million (ppm). 

Microstructural examinations revealed that the cracks originated on the external surface (Fig. 9.15). The cracks were highly branched and transgranular. The branched, transgranular character of these cracks is typical of stress-corrosion cracking of austenitic stainless steels. The thick-walled fracture faces are also typical of cracking by this mode. 

Microstructural examinations disclosed highly branched, predominantly transgranular cracks originating on the internal surface. Cracks of this form are typical of SCC in austenitic stainless steels. 

Poor Weldability a. Underbead cracking, high hardness in heat-affected zone. b. Sensitization of nonstabilized austenitic stainless steels. a. Any welded structure. b. Same a. Steel with high carbon equivalents (3), sufficiently high alloy contents. b. Nonstabilized austenitic steels are subject to sensitization. a. High carbon equivalents (3), alloy contents, segregations of carbon and alloys. b. Precipitation of chromium carbides in grain boundaries and depletion of Cr in adjacent areas. a. Use steels with acceptable carbon equivalents (3) preheat and postheat when necessary stress relieve the unit b. Use stabilized austenitic or ELC stainless steels. 

Austenitic stainless steels (the 300 series) are particularly su-sceptible to stress-corrosion cracking. Frequently, chlorides in the process stream are the cause of this type of attack. Remove the chlorides and you will probably eliminate stress-corrosion cracking where it has been a problem. 

Some of the most obvious examples of problems with gas and materials are frequently found in refining or petrochemical applications. One is the presence of hydrogen sulfide. Austenitic stainless steel, normally a premium material, cannot be used if chlorides are present due to intergranular corrosion and subsequent cracking problems. The material choice is influenced by hardness limitations as well as operating stresses that may limit certain perfonnance parameters. 

Eliminate unfavorable environments. The presence of oxygen and other oxidizers is a critical factor in stress corrosion cracking. For example, the cracking of austenitic stainless steel in chloride solutions can be reduced or completely eliminated if oxygen is removed. 

The corrosive environments which cause SCC in any material are fairly specific, and the more common combinations are listed in Table 53.2. In the case of chloride stress corrosion cracking of the 530 series austenitic stainless steels it is generally considered that the risk is... 

Boiler salts can contain chloride ions. When carried over into the steam (e.g. during priming) this can result in chloride stress corrosion cracking of austenitic stainless steel expansion bellows. In steam systems where freedom from chloride cannot be guaranteed, bellows... 

Steel is the most common constructional material, and is used wherever corrosion rates are acceptable and product contamination by iron pick-up is not important. For processes at low or high pH, where iron pick-up must be avoided or where corrosive species such as dissolved gases are present, stainless steels are often employed. Stainless steels suffer various forms of corrosion, as described in Section 53.5.2. As the corrosivity of the environment increases, the more alloyed grades of stainless steel can be selected. At temperatures in excess of 60°C, in the presence of chloride ions, stress corrosion cracking presents the most serious threat to austenitic stainless steels. Duplex stainless steels, ferritic stainless steels and nickel alloys are very resistant to this form of attack. For more corrosive environments, titanium and ultimately nickel-molybdenum alloys are used. 

The shape of a vessel determines how well it drains (Figure 53.7). If the outlet is not at the very lowest point process liquid may be left inside. This will concentrate by evaporation unless cleaned out, and it will probably become more corrosive. This also applies to horizontal pipe runs and steam or cooling coils attached to vessels. Steam heating coils that do not drain adequately collect condensate. This is very often contaminated by chloride ions, which are soon concentrated to high enough levels (10-100 ppm) to pose serious pitting and stress corrosion cracking risks for 300-series austenitic stainless steel vessels and steam coils. 

Nickel-chromium alloys can be used in place of austenitic stainless steels where additional corrosion resistance is required. These alloys are still austenitic but are highly resistant to chloride-induced stress corrosion cracking when their nickel content exceeds 40 per cent. 

D ye penetration inspection. This is a simple technique, requiring a minimum of operator training. In the hands of a skilled operator, it is capable of detecting fine cracks such as chloride stress corrosion cracks in austenitic stainless steels and fatigue cracks. 

In practice, by far the most common case of stress corrosion is that occurring when austenitic stainless steels are simultaneously exposed to tensile stresses and hot, aqueous, aerated, chloride-containing environments. In this case the major variable is alloy composition and structure virtually all austenitic stainless steels are more or less susceptible to stress-corrosion cracking in these environments, while ferritic and ferritic/austenitic stainless steels are highly resistant or immune. 

Rhodes, P. R., Mechanism of Chloride Stress-corrosion Cracking of Austenitic Stainless Steel , Corrosion, 25, 462 (1969)... 

Wilde, B. E. and Kim, C. D., The R61e of Hydrogen in the Mechanism of Stress-corrosion Cracking of Austenitic Stainless Steel in Hot Chloride Media , Corrosion, 28, 350 (1972) Lin, F. and Hochman, R. F., Electrochemical Study of Stress-corrosion Cracking of Ti 8-1-1 Alloy and NaCl Solutions , Corrosion, 28, 182 (1972)... 

The fracture mode of stress-corrosion cracks in austenitic stainless steels can be transgranular, intergranular or a mixture of both. One of the earliest environments found to cause problems was solutions containing chlorides or other halides and the data due to Copson (Fig. 8.30) is very informative. The test solution for that data was magnesium chloride at 154°C the alloys contained 18-20alloy with a composition of approximately 18Cr-8Ni has the least resistance to cracking in this environment. 

Austenitic stainless steels will exhibit stress-corrosion cracking in hot aqueous chloride solutions, in acid chloride containing solutions at room temperature, in hot caustic solutions and in high-temperature high-pressure oxygenated water. 

Fig. 8.31 Effect of element shown on resistance of austenitic stainless steels to stress-corrosion cracking in chloride solutions (after Sedriks )...

The behaviour of austenitic stainless steels in caustic solutions has received less attention than cracking in chloride environments. Transgranular cracking has been reported for low-carbon (< 0.05%) steels in caustic solutions, whereas higher carbon content alloys cracked intergranularly. Wilson and Aspen showed that resistance to cracking was not decreased by sensitisation heat treatments. Type 316 stainless steel has been shown to be more susceptible to cracking in caustic than type 304. ... 

Much of the recent research on stress-corrosion cracking of austenitic stainless steels has been stimulated by their use in nuclear reactor coolant circuits. The occurrence of stress-corrosion cracking in boiling water reactors (BWR) has been documented by Fox . A major cause for concern was the pipe cracking that occurred in the sensitised HAZ of the Type 304 pipework, which is reported to have been responsible for about 3% of all outages of more than 100 h from the period January 1971 to June 1977. 

James, L. A., Fatigue crack propagation in Austenitic stainless steel . Atomic Energy Review 14, 37-86 (1976)... 

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