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DIFFRACTION NOTES      5521 FAIR LANE, CINCINNATI, OH 45227      (513) 561-0883      No. 41      Fall 2014

Using LPB to Inhibit Stress Corrosion Cracking of Welded Stainless Steel Components

Stress corrosion cracking (SCC) is one of the most serious metallurgical problems faced in many industries. SCC has been observed for decades in low carbon austenitic alloy weldments such as types 304L and 316L stainless steel and continues to be a primary maintenance concern for many components. Despite the fact that these alloys generally exhibit excellent corrosion resistance, studies have revealed that all grades and conditions of austenitic stainless steels are susceptible to SCC given the right conditions, namely a corrosive environment and applied or residual tensile stress above a certain threshold [1]. The threat of SCC damage directly impacts inspection, repair and replacement costs. It is a particularly dangerous and potentially catastrophic mechanism that initiates slowly and can progress undetected at stresses well within engineering design limits and typical operating conditions. A cost effective means of mitigating SCC would greatly reduce operational and maintenance costs.

Lambda Research conducted a study, funded through a DOE SBIR, on the effects of beneficial compressive residual stress on the prevention of SCC in welded stainless steel components. It was shown that post-weld surface enhancement processing via Low Plasticity Burnishing (LPB) [2-4] can be used to introduce deep compression into tensile weld zones thereby mitigating SCC.

MATERIAL PROCESSING

Type 304L and 316L stainless steel was chosen for the study due to its widespread application. Plate material conforming to ASME SA240 was machined and welded into test plates of approximately 1 x 6 x 12 in. Welding was performed by a certified nuclear repair facility using a shielded metal arc welding (SMAW) process and weld filler metal E308 and 152 for the 304L and 316L plates, respectively.

One half of the weld was LPB processed to impart a depth and magnitude of compression necessary for SCC mitigation. A close up picture of the weld region showing LPB treated and untreated material can be seen in Figure 1. As can be seen in the figure LPB processing improves the surface finish and reduces surface irregularities.

RESIDUAL STRESS AND STRESS CORROSION CRACKING TESTS

X-ray diffraction residual stress measurements were performed on the 304L and 316L welded test plates. Measurements were made at locations adjacent to the weld and at distances of 0.2, 0.5, and 1 in. from the weld fusion line on both the LPB treated and untreated halves of the specimens. Figure 2 shows the residual stress distributions plotted as a function of depth for each of the various distances from the weld fusion line. Peak tension of over +100 ksi was measured in the untreated side of both specimens. LPB treatment produced compression approaching -100 ksi.

SCC tests were performed using hot magnesium chloride (MgCl2) solution exposure. The use of MgCl2 solution exposure is a generally accepted method for determining the susceptibility of materials to SCC and is described in ASTM G36 [5]. The test consisted of a 32 hour exposure at a temperature of 120C. Figure 3 contains post-exposure pictures of a test sample. Fluorescent dye penetrant was used to aid in the visual inspection of cracks. Both the 304L and 316L alloy samples showed similar results. The LPB treated sides of the plates were completely free of any SCC. Significant SCC was observed along the untreated side of the weld fusion line and along the edge of the plate on the untreated side. The cross section images illustrate the depth of the SCC. Crack depths on the untreated side of the plate approach the weld depth.

Summary:

  • Heat affected zones of welded 304L and 316L stainless steel exhibited tensile residual stress of over +100 ksi.

  • LPB processing successfully imparted deep residual compression approaching -100 ksi.

  • SCC tests reveal severe cracking in the untreated material with depths approaching that of the entire weld.

  • No SCC occurred on the LPB treated side of any of the welded specimens.

  • LPB was shown to effectively eliminate stress corrosion cracking in welded 304L and 316L stainless steel.

REFERENCES

[1] P. L. Andresen and M. M. Morra (2008) Stress Corrosion Cracking of Stainless Steels and Nickel Alloys in High-Temperature Water. Corrosion: January 2008, Vol. 64, No. 1, pp. 15-29.

[2] Navy Phase II SBIR (2002) Contract N68335-02-C-0384, "Affordable Compressor Blade Fatigue Life Extension Technology"

[3] Air Force Phase II SBIR (2003), Contract F33615-03-C-5207, "Component Surface Treatments for Engine Fatigue Enhancement"

[4] NASA Phase II SBIR (1999) Contract NAS3-99116, "Low Cost Surface Enhancements Method for Improved Fatigue Life of Superalloys at Engine Temperatures"

[5] ASTM Standard G36, 1994 (2013), "Standard Practice for Evaluating Stress-Corrosion-Cracking Resistance of Metals and Alloys in a Boiling Magnesium Chloride Solution," ASTM International, West Conshohocken, PA, 2003, DOI: 10.1520/G0036, www.astm.org.

ANNOUNCEMENTS

Optimizing Manufacturing

Residual stresses resulting from all aspects of manufacturing have a substantial effect on the fatigue properties of your component. Lambda provides the tools and expertise to optimize your manufacturing processes to produce optimal residual stress levels for peak component performance. For more information click here or contact Perry Mason.

Retained Austenite Measurement

Hardening of steels requires heating to an austenitic phase and quenching to room temperature to produce a hard martensitic phase. Due to incomplete transformation, some austenite is retained at room temperature. Retained austenite can have a detrimental affect on the mechanical properties of the steel. Lambda uses dedicated Bragg-Brentano diffractometers designed for retained austenite measurement to provide accurate retained austenite measurement in strict adherence to the ASTM E975 and SAE SP-453 specifications. Click here for more information or contact Chris Barger.

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