African Fusion August 2016

Repair of graphitised pipe welds

No other tensile properties were affected by the presence of HAZ graphite. The impact toughness was, on average, 218 J for the weld metal. The average HAZ toughness was 137 J. No other mechanical properties (such as the hot tensile strength, the bend angle, or the hardness) were affectedby the presence of HAZ graphite.

Sample number

Yield strength (MPa)

Tensile strength (MPa)

Location of failure

263 245 268 269 259 277 272 277

410 416 416 418 418 419 451 450

PM1

1

PM PM PM PM PM

2

Figure 7: Effects of prolonged heat treatment (at 635°C) on the tensile strength of materials and the degree of spheroidisation as described by Toft and Marsden [12].

3

HAZ HAZ

4

Table 2: Tensile results as measured using transverse tensile samples with a rectangular cross-section.

Sample number

Yield strength (MPa)

Tensile strength (MPa)

Location of failure

407 410 420 412 415 412 452 447

70 69 70 71 70 44 74 69

PM PM PM PM PM

1

2

3

HAZ

Figure 8: Step-like fracture of rectangular tensile test coupon transverse to as-received circumferential weld (sample 4).

PM PM

4

Table 3: Tensile results as measured using transverse tensile samples with a round cross-section.

Figure 9: Higher magnification image of the fracture surface visible in the lower left hand corner of the previous image (sample 4). the prolonged heat treatment were spheroidised. No graphi- tisation during the extended heat treatment at 635°C was observed, consistent with the published work ([10], Figure 3). A cross section of the graphitised fracture surface can be seen in Figure 8 and Figure 9. The fracture plane seems to develop by alternating rupture through the plane defined by the graphite nodules and tearing through the ferrite-pearlite structure that does not contain graphite. Figure 10 is a high magnification image of a graphite par- ticle etched from the metallic substrate (4%Nital for five min- utes), showing the complex substructure of a graphite nodule. Conclusions • The presence of HAZ graphite observed in this study hadno effect on themechanical properties of thematerial, except for the decrease in reduction in area when the tensile test

Tensile test results showed that all samples that experi- enced prolonged heat treatment displayed a reduction in ten- sile strength of 30 to 40 MPa. The reduction in tensile strength can be explained by pearlite spheroidisation during prolonged exposure at 635°C – Figure 7. All samples that experienced Figure 6: Failure of sample 4 through the graphitised HAZ. a) the fracture profile in relation to weld metal. b) the fracture surface (original magnification 8×).

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August 2016

AFRICAN FUSION