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WELDING RESEARCH

A

B

Fig. 7 — Applied da/dN-DK curves for various grain sizes. A — 316L; B — AL6XN.

Table 4 — Summary of DK Levels below Which Crack Closure Was Observed

Grain Size (mm)

DK below which closure was observed, (MPam)

24 3.3 103 12.8 147 21.6

21 2.4 211 26.3 281 29.7

316L Stainless Steel No closure observed 7 · 10 2 · 10 AL6XN Stainless Steel No closure observed 3 · 10 6 · 10 –10 –9 –10 –10

316L–As received

24 3.3

316L–Annealed 45 min.

103 12.8

316L–Annealed 5 h

147 21.6

AL6XN–As received

21 2.4

AL6XN–Annealed 45 min

211 26.3

AL6XN–Annealed 5 h

281 29.7

320-C

8

306-M

239-C

13

227-C

15

397-M

10

282-C

23

275-C

25

Table 5 — Summary of Grain Sizes and Calculated Yield Strengths

for 316L Stainless Steel

Sample

d (mm)

sys (MPa)

M-Measured C-Calculated

DK below Which Grain Size Effects Are Expected,MPam

to changing their path. The expected re- sult would be a tortuous crack path, as ob- served experimentally in this study.

Equation 1 can be used with known sys values to estimate the DK value below which these grain size effects are expected to occur. This value of DK is denoted at DKGS for reference. By setting the plastic zone size (given by Equation 1) equal to the grain size, the DK value below which grain size ef- fects are expected to occur is given as

1

DK

GS

=s

ys

Ø d ø2 Œœ

Œ0 0 3 3 œ . ß º (2)

Where d is the grain size. There was in- sufficient material available to directly de- termine the yield strength of all the sam- ples as a function of grain size. However, knowledge of the yield strength of the as- received 316L and AL6XN provide two useful data points. In addition, Hall-Petch parameters established for the 316L alloy

permit a good estimate of the yield strength as a function of grain size for this alloy. Priddle (Ref. 21) previously estab- lished the influence of grain size on yield strength with the following Hall-Petch equation for 316L stainless steel:

s

ys

s o = +

k

d d

(3)

in which so = 163 MPa and kd = 0.77 MPa m. Equation 3 produces very good agreement between calculated (320 MPa) and measured (306 MPa) sys values for 316L in the as-received condition (5% error). Although no Hall-Petch relation was available in the literature for AL6XN, Equation 3 can be used to at least estimate the expected change in yield strength with grain size for AL6XN. Here, it is assumed that the incremental change in sys with d is similar to 316L (i.e., the kd constant in Equation 3 is identical), and that the net variation in sys can be accounted for by the

so term in Equation 3. With this assump- tion, the so term in Equation 3 can be de- termined so that agreement is found be- tween the starting grain size (d = 21 mm) and yield strength (sys = 397 MPa) of AL6XN. A so value of 229 MPa provides this agreement. Thus, the following two Hall-Petch equations were used to deter- mine yield strength as a function grain size

s

ys

= 163 +

0.77 d

(4)

for 316L stainless steel

s

ys

= 229 +

0.77 d

(5)

for AL6XN stainless steel

w h e r e d i s i n m m . T a b l e 5 s u m m a r i z e s s y s v a l u e s c a l c u l a t e d f o r e a c h g r a i n s i z e f each alloy. Also shown in the table are the DKGS values below which grain size effects o r

12 -S JANUARY 2004

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