Table 1 — Chemical Compositions of Base Metals and Filler Metals
Note: All values expressed in wt-%.
Fig. 1 — Schematic illustration of C(T) specimens.
gated, and crack closure measurements were obtained through a novel compli- ance offset method. The increase in fa- tigue crack growth rate that occurred as the stress ratio increased from 0.10 to 0.55 was attributed to an extrinsic crack closure effect in which higher stress ratios pro- moted a fully open crack and correspond- ing higher growth rates. Continued in- crease in the crack growth rate that occurred as the stress ratio increased fur- ther from 0.55 to 0.70 was attributed to a true intrinsic material response to increas- ing stress ratio. These results were useful because they provided a critical stress ratio needed to overcome crack closure ef- fects associated with residual stress.
The purpose of the current research is to use a constant DK test procedure in order to determine the influence of mi- crostructure on the fatigue resistance of welds relative to that of the base metal. Once this relative relation was estab- lished, full fatigue curves over a larger DK range were established for base metals with well-controlled, uniform microstruc- tures in order to investigate the influence of microstructure on fatigue behavior in more detail. The results of this research shed light on the role of weld metal mi- crostructure on fatigue crack growth rate.
Materials and Welding Procedure
The compositions of the base metals and filler metals used in this study are
summarized in Table 1. Details of the welding and sample preparation tech- niques can be found in Ref. 12 and will be briefly described here. Gas metal arc welds (GMAW) were prepared with matching filler metals on each alloy as de- scribed in previous work (Ref. 12). It should be noted that matching filler metal for Alloy AL6XN is not typically used in industrial practice. This alloy is typically welded with a nickel-based filler metal en- riched in Mo (Ref. 13) to help compensate for Mo microsegregation. However, the objective of this work was to investigate the influence of microstructural variations between the base metal and filler metal at similar compositions. Thus, a special heat of matching AL6XN filler metal was pre- pared and used for this purpose. Multiple passes were deposited on 19-mm-thick base metals using an automatic welding system with 1.6-mm-diameter filler metal and a wire feed speed of 470 cm/min for the 316L weld and 521 cm/min for the AL6XN weld. The arc current was 280 A, the voltage was approximately 25 V, and the travel speed was 27–33 cm/min for the 316L weld and 41–46 cm/min for the AL6XN weld. All welding was conducted in the flat position using a 98Ar/2O2 shielding gas mixture with no preheat and a 150°C interpass temperature. Five layers were used to fill the weld joints with a total of 14 passes.
Both AL6XN and 316L base metal samples were subjected to heat treatments with the intent of increasing the grain size in order to determine the influence of
grain size on fatigue crack growth in a con- trolled manner. Samples were wrapped in stainless steel foil to minimize oxidation during heat treatment and heated at 1250°C for either 45 minutes or 5 hours. The samples were air cooled to room tem- perature after heat treating. Grain size was measured in accordance with ASTM Stan- dard E112 (Ref. 14). Compact tension (C(T)) specimens were machined from the base metals and welds as shown schemati- cally in Fig. 1. The samples conformed to requirements of ASTM Standard E647 (Ref. 15). A fatigue crack starter notch 2.54 cm in length, with a 0.15 mm diame- ter, and a 0.077 mm radius of curvature, was inserted into the specimen by wire electrical-discharge machining (EDM). As previously described, the starter fatigue notch in the welds was placed perpendicu- lar to the welding direction.
Fatigue Crack Propagation Testing
Details of the testing procedure can also be found in Ref. 12 and will be briefly reviewed here. All testing was conducted in accordance with ASTM E647 (Ref. 15). An automated, computer-controlled test system was used for testing, acquisition, and reduction of data. Testing was first performed on the welds using a constant DK test procedure at a constant R ratio of 0.60. The DK level was held constant at 15 MPa√m for the 316L weld and 8 MPa√m for the AL6XN weld. With this method, an algorithm is used to reduce the loads as the crack grows so that the stress intensity range remains constant. This stress ratio of 0.60 was utilized based on earlier work (Ref. 12), which demonstrated that this R value effectively overcomes residual stress effects. Thus, any observed change in fa- tigue resistance can be attributed to mi- crostructural effects. The fracture surface