Crack Paths 2006

Figure 11: ]/[105/8cyclemdNda ˜|

d N d a ˜ |

Figure 13:

] / [ 1 0 2

/

1 0 c y c l e m

Finally, fracture toughness tests have been performed for all the materials; the obtained

results are reported in Table 6 where both KQ and K Max were reported together with the

ratio PMax on PQ. Each value is evaluated as the average value between three

experimental values.

Table 6. experimental values obtained from fracture toughness tests.

Material code ] [ m M P a K Q Q M a x P P

] [ m M P a

K

Max

27.0

1.14

Cr1

30.8

20.8

1.11

Cr2

23.0

26.1

1.08

Cr3

28.2

From the analysis of the data, Cr1 shows both a KQ and KMax higher than Cr2 and Cr3.

For Cr1 and Cr2 series, the ratio PMax on PQ is not able to respect the limit 1.1 of the

A S T ME 399 Standard [16].

C O N C L U D IRNEGM A R K S

In this paper, results on the effects of pores characteristics and metallographic phases on

the fatigue crack initiation and propagation in Chromium pre-alloyed P Msteels were

reported. On the basis of these results, the following conclusions can be made

- the Paris law crack growth exponent is around 6.0 for 1120°C sintered and around

4.7 for 1250°C sintered steels

- about the steels sintered at 1120°C, observing the crack growth curve, at a given

value of 'K, the crack growth rate in Cr2 steel is higher than Cr1. This can be

related to the different microstructure due to the different cooling rate from

1120°C sintering temperature: martensite and transforming austenite in Cr2 steel

and perlite and martensite in Cr1. Cr1 and Cr2 steels have similar m-coefficient,

but different C-coefficients of the Paris law.

-

about the fatigue crack initiation,

neither the pores characteristics

nor

metallographic phases seems to strongly influence the stress intensity threshold

value.