Stress Corrosion Cracking in Welded 316SS Screener Shaft

S. Kaewkumsai, S. Aumparn, and E. Viyanit

National Metal and Material Technology Center (MTEC),

National Science and Technology Development Agency (NSTDA),

114 Thailand Science Park, Paholyothin Rd., Klong 1,

Klong Luang, Pathumthani 12120 THAILAND

Phone 66-2564-6500 ext. 4736, Fax.66-2564-6332, E-Mail: siamk@mtec.or.th

ABSTRACT

This paper described the failure of stainless steel shaft grade AISI 316, which has been investigated after being in service for nearly 3 years. It failed by stress corrosion cracking, which was originated at the welded zone. The investigation included visual examination, optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), spectrometry, and metallography. Analytical results revealed that crack was propagated at the outer surface of the welded zone. Significant evidence of pitting was observed in the originated zones which results from the corrosion attacked. It is necessary to check the material properties after welding process and one should ensure that the residual tensile stresses is prohibited.

KEY WORDS: Stress Corrosion Cracking; Stainless Steel; Branched Crack

1. Introduction

Stress corrosion cracking (SCC) is a term used to describe failures in engineering parts that result from environmentally assisted cracks propagation. This phenomenon is associated with a combination of stresses above some threshold values, specific environment and sensitive material, which leads to surface cracks especially in passive film forming metal [1]. Austenitic stainless steel parts were frequently subject to failure from stress corrosion cracking in chloride containing environments. To induce SCC, a high chloride concentration is required, although relatively small amount of chloride is sufficient on heated surfaces, area where chloride concentration can occur, or where chloride is concentrated by pitting or crevice corrosion. The cracking continues at low stresses and commonly occurs as a result of residual stresses from welding or manufacturing. The cracking is normally transgranular, although it may switch to intergranular mode as a result of sensitization of the steel.

In service, welding residual stresses can superpose on applied stress and if being tensile in nature, they may promote SCC. For on-site application, the maximum residual stresses produced in the welding process may be higher than the yield strength of material [2]. Since the breakdown of passive films has been recognized as playing a key role in the pitting and SCC of austenitic stainless steels, it is reasonable to correlate the effects of microstructure and residual stress on corrosion resistance to the properties of the passive film.

The failed material for failure analysis was the stainless steel screener shaft, which had been in service for nearly 3 years. The shaft was made of commercial stainless steel grade AISI 316. The shaft was used for vibrated the screener of plastic powder. The vibration amplitude and frequency cycles were controlled around 3 mm and 1000 cycle/second, respectively. The temperature in serviced was exposing around 80-90 ºC. The shaft was fabricated by welding process without preheat and post-heat at the welded location and fracture was found in this area. The fractured shaft for failure analysis is shown in Fig.1.

Fig.1 As- received screener shaft for failure analysis and fracture site

2. Investigation Methods and Results

2.1 Visual Examination

As-received fractured shaft was thoroughly examined visually with the aid of a stereo microscope. The shaft had fractured into two pieces. A portion of one of the mating fracture surfaces was cut away from the shaft and used for fractography. The fracture surface, which cut from as received failed shaft, was carefully examined both visually and the aid of a stereoscopic microscope (up to 10X). The fracture surface on the shaft shows the macro-view in Fig. 2a. The fracture surface was displayed a flat region normal to the axis on the shaft. The prominent array of radial marks indicative of brittle fast fracture was visibly observed (Fig. 2b). The crack originated on the outside surface of the shaft nearby the welded area. It emanated from the surface pit. Examination of the outside cylindrical surface of the shaft at the fracture origin site revealed a rust color scale and evidence of a small hole.

Fig. 2: Fracture surface of the failed part a) entire fracture surface b) radial marks radiated from the origin

2.2 Fractography with SEM

The fracture initiation site in the fractured specimen was examined in the scanning electron microscope (SEM). SEM examination of the surface of fracture origin area revealed a branched crack generated from the fracture origin and also was covered by the corrosion products as shown in Fig. 3a. The Energy Dispersive Spectrometry (EDS) was used to determine the chemical composition analysis of contaminant particles on the surface of fracture origin. The results of the corrosion products in a fracture origin area revealed the high peak of chlorine (Fig. 3b).

Fig.3a SEM fractograph of the fracture origin surface revealed a branched crack and was covered by corrosion products b) EDS spectrum of the corrosion products on in the area of fracture origin

2.3 Composition Analysis

A piece of shaft was subjected to spark emission spectrometer to determine the bulk composition of material. The chemical composition of the test sample is close to specification of AISI 316.

2.4 Cross-sectional and Microstructure Analysis

A sample was cut from the section near the fracture surface. The sample was prepared for examination by mounting, polishing, and etching with glyceregia solution. The microstructure in the base metal was generally found to be austenitic structure (Fig. 4a). The welded metal shows the dendritic structure, which was resulted from fast cooling rate. The microstructure characteristic near the fusion region is presented in Fig. 4b. Note the slag inclusions at the weld interface were found. A branched crack was found adjacent to base metal and welded metal. The crack morphology is typical of stress corrosion cracking.

Fig.4: a) Normal structure b) cracks at the weld interface

2.5 Hardness Testing

Micro-hardness measurements were taken across the fusion line region that included base metal and weld metals, which had carried out on metallographic sample. The hardness values of the welded metal were found to be a little greater than that of base metal. The hardness values are not shown significant information.

3. Discussions

Visual and macro-examination and SEM fractography clearly established that the screener shaft failed in mode of brittle fracture as indicated by the radial mark. The origin was located at a localized region on the outside cylindrical surface of the shaft. It was occurred nearby the toe of welded area that contained a small hole of slag inclusions and branched-cracks, which act as stress concentrators and fracture origin sites. SCC in austenitic stainless steels is easily recognized by the branched nature of transgranular cracks [3]. EDS analysis of the impurities on a small hole revealed that it contained a large amount of chlorine, an element that contributed to crack initiation. Chlorides are the big problem when using the 300 series grades of stainless steel. The combination of the residual stress, the presence of chlorine, and cyclic load makes the material susceptible to fracture.

Normally, stainless steel has a protective film that resists corrosion damage from chemical species. For the failed part, the defects generated from rust caused discontinuities in protective film. Welding often makes this situation worse, prone to the metallurgical alters and residual stresses introduced. Therefore, the failure of the shaft was probably caused by the interaction between the residual stress and chloride containing media at the welded areas followed by crack initiation and propagation. Suresh M. [4] said that the temperature usually needs to be above 70 ºC before SCC can occur. In this case, the service temperature was operated at 80-90 ºC, the ideal for generated SCC. The higher temperature, the higher concentration of chloride promote to occurrence of SCC.  In service, welding residual stresses can superpose on applied stresses and, the shaft being tensile in nature, they may promote SCC. In engineering practice, the maximum residual stresses produced in welding process may be higher than the yield strength of material, so that they can induce SCC without the aid of applied stresses.

4. Conclusion

The screener shaft was failed by stress corrosion cracking which was induced by chloride contamination in the service environment and the presence of residual stress. Stress relieve of the shaft after welding is necessary. Periodic cleaning of contaminants on the surface of the shaft is also recommended to avoiding such failure.

References

[1] Manfredi C. et al, “Failures by SCC in buried pipelines”, Eng Fail Anal, Vol.9, 2002, pp.495-509.

[2] Lu B.T., “Pitting and stress corrosion cracking behavior in welded austenitic stainless steel”, Electrochemica Acta, Vol.5, 2005, pp.1391-1403.

[3] Lynch S.P., “Failures of Structures and Components by Environmentally Assisted Cracking”, Failure Analysis Case Studies, Eng Fail Anal, Vol.1, 1994, pp.77-90.

[4] Suresh M. et al, “Failure Analysis of Stainless Steel Pipeline”, Eng Fail Anal, 2008, pp.497-504.