results are aligned with the fatigue results so that the variation in surface roughness can be matched with the corresponding crack growth rates. A length scale key is provided on the right of Fig. 3C for refer- ence. These results clearly show that the fatigue resistance of the weld metal is bet- ter than that of the base metal and the fracture surface roughness increases sig- nificantly as the crack propagates from the base metal into the weld.
The results shown in Figs. 2 and 3 sug- gest that the large grain size is responsible for the rough fracture surface and con- comitant improvement in fatigue resis- tance. However, the large variation in grain size and columnar grain morphology in the weld make definitive conclusions difficult. Thus, base metal samples with controlled variations in grain sizes were used for fatigue testing to investigate this potential effect in more detail and to de- termine how grain size influences crack growth rates over a larger applied stress intensity range.
Influence of Grain Size on Fatigue Resistance
Table 2 and Fig. 4 summarize the influ- ence of annealing time on the grain size of 316L and AL6XN stainless steels at 1250°C. Each alloy exhibits similar start- ing grain sizes. With annealing at 1250°C, the AL6XN grain size is consistently higher at each annealing time. This may be attributed to the small amount of ferrite present in the 316L base metal, which would pin grain boundaries and limit grain growth. The AL6XN alloy, by comparison, is fully austenitic and therefore contains no second phases to restrict grain growth.
Standard fatigue crack growth data, along with DKeff data for five slope offset levels, are provided in Figs. 5 and 6 for the base metals of varying grain sizes. The DKeff curves are analyzed in the same manner as discussed in previous research in detail (Ref. 12). Briefly, the presence of unique curves for each slope offset level
5B) and 2 · 10–9 indicates that crack closure is occurring while a single, coin- cident curve repre- sents a fatigue crack that is fully open. For example, the fatigue results for 316L tested in the as-received condition with a grain size of 24 mm (Fig. 5A) show all offset curves are coincident for all the offset slope lev- els (which gives the appearance of a single curve) over the entire range of da/dN, indicating that crack propaga- tion has occurred free of closure for all DK levels. In this case, the applied DK and effective DK are equivalent since the crack is always fully open. By compar- ison, the results generated on 316L base metal with grain sizes of 103 mm and 147 mm (Fig. 5B and C) exhibit crack closure up to approximately 7 · 10–10 m/cycle (Fig. spectively. (The range of da/dN where clo- sure occurs is indicated in each figure.) Thus, as grain size increases, crack closure effects become evident at higher crack growth rates and corresponding DK val- ues. Similar effects are observed for the AL6XN alloy in Fig. 6, although the influ- ence of grain size on crack closure is not as large as that observed in 316L stainless steel. In this case, crack closure is ob- served below approximately 3 · 10–10 m/cycle for the AL6XN sample with a 210- mm grain size (Fig. 6B) and 6 · 10–10 m/cycle for the sample with a 280-mm grain size — Fig. 6C. The unique offset curves in Fig. 6B are difficult to identify from the figure, but direct inspection of m/cycle (Fig. 5C), re- Fig. 4 — Grain size as a function of annealing time at 1250°C for 316L and AL6XN stainless steel base metals. the corresponding numerical data indi- cates an appreciable level of crack closure in this growth rate range. Figure 7 summarizes the applied da/dN-DK curves for the various grain sizes of each alloy. Two fatigue curves were produced for each base metal in the starting condition (smallest grain size) in order to demonstrate reproducibility of the test results. The data in this figure demonstrate the significant influence of grain size on fatigue resistance. Specifi- cally, as the grain size increases, the crack growth rate decreases for a given applied stress intensity range. The reduction in crack growth rate with increasing grain size is particularly evident at low levels of applied stress intensity range near the threshold regime. The crack growth rates then become similar as the applied stress intensity range and concomitant crack growth rates increase to high values. The DKGS values noted in Fig. 7 will be dis- cussed in the next section.
AL6XN–Annealed 45 min.
AL6XN–Annealed 5 h
316L–Annealed 45 min.
316L–Annealed 5 h
Table 2 — Summary of Grain Size Measurements for 316L and AL6XN Stainless Steels after Heat Treating at 1250°C
Number of Data
Average Grain Size (mm)
Table 3 — Summary of DKth Values and Number of Data Points Utilized for DKth Calculations
and 10–9 m/cycle
As-received Annealed: 1250°C – 45 minutes Annealed: 1250°C – 5 hours