Solidification cracks occur in the mushy zone of a weld bead on cooling, as the strains that develop exceed the ductility of the (solid + liquid) mixture. The modern theory of solidification cracking was developed by Borland,2 who highlighted the impor­tance of the quantity and distribution of the liquid near the terminal phase of solidification, as well as the stresses that act on that liquid. The primary factors that affect hot cracking are summarized in texts by Kou, Messler, and others.3-6 These factors are listed below:

1. The solidification temperature range: The larger the solidification temperature range, the more exten­sive the solid + liquid mushy zone, which is sus­ceptible to cracking. While large solidification temperature ranges may promote crack healing via

Homologous temperature (T/Tmelt)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Weld cracking Subsolidus Supersolidus Segregation-induced cracking: hydrogen Precipitation-induced cracking: also known as ductility dip cracking, strain-age cracking, reheat cracking, subsolidus cracking Liquation -type

‘Hot tearing’	Solidification
(mechanical)	-type

Environmental degradation

Impurity segregation via diffusion

Ordering reactions/brittle second phases precipitate

Radiation-induced segregation Radiation hardening Creep-rupture Liquid and solid metal embrittlement

Figure 1 Comparison of the typical temperature ranges for the different types of weld cracking (top) and forms of environmental degradation common to nuclear power systems (bottom). All temperature ranges are approximate, based on the homologous temperature of the alloy under consideration.

500 pm

Figure 2 Illustration of the different forms of hot cracking. Liquation cracks occur in the partially melted and/or heat-affected zone (HAZ) of the weld bead being deposited (top). Solidification-type cracks occur in the composite region of the weld during solidification (middle), and hot tears are dominated by mechanical forces and occur at macroscopic notches (bottom). Note that the cracks are all from nickel-chromium alloy welds but are not all from the same weldment.

backfilling, solute-rich ‘backfill’ may have degraded properties relative to the bulk weld metal. The approximate solidification temperature ranges of several alloys used in nuclear construction are shown in Figure 3.

2. The solidification path: Solidification crack suscep­tibility is markedly influenced by the type and distribution of solid phases, for example, initial 8-ferrite formation from the liquid in austenitic stainless steel welds imparts hot crack resistance

by breaking up the solidification structure and by scavenging tramp elements (e. g., sulfur and phosphorous). Conversely, eutectic-type reac­tions during terminal solidification (e. g., liquid! g + Laves in nickel-based alloys) are notably detrimental to solidification cracking resistance.7 Figure 4 illustrates the calculated solidification path and hot cracking resistance of two nickel — chromium filler metals. The more solute-rich filler metal that forms Ni2(Nb, Mo)-type Laves phase is

1900

§ 1600

К

1500

03

0

0 1400

о

сЛ "О

1300

_оЗ

=3

о

О 1200 1100 1000

Figure 3 Comparison of the calculated solidification temperature ranges of some common materials used in nuclear power systems (JMatPro, Version 4.1). For a given alloy class, hot cracking is promoted by larger solidification temperature range and low solidus temperature. Note that the compositions used are ‘typical’ values and significant variability exists within each alloy’s specification range.

much more prone to hot cracking than the Laves — free alloy.

3. The surface tension of the terminal liquid: Low surface tension liquids wet solidification boundaries and promote cracking by increasing the amount of interface incapable of supporting appreciable ten­sile strains.

4. The metallurgical structure of the weld: Large colum­nar dendritic grains are more susceptible to solid­ification cracking than finer equiaxed structures. Coarse solidification structures result in longer crack paths and less grain boundary area to dis­tribute elements that lower the solidus and/or embrittle the boundary. Columnar grains may exacerbate hot cracking by promoting wetting of the grain faces and can result in linear solidifica­tion boundaries near the centerline of the weld bead where tensile stresses are often the highest.

5. The mechanical forces that act on the weld: High tensile strains during the terminal stages of solidification promote cracking.

Given the complexity of the factors that contribute to

solidification cracking, it is difficult to predict its

occurrence in production welds. However, weld­ability tests such as the transvarestraint test enable a quantitative ranking of alloys with respect to solidification cracking susceptibility and offer a standardized methodology to optimize welding para- meters.8-10 Results from transvarestraint tests on sev­eral corrosion-resistant alloys are shown in Figure 5, which compares the maximum crack distance (i. e., the extent of the mushy zone when the crack distance becomes insensitive to the applied strain) and hence the intrinsic susceptibility of the alloy to solidifica­tion cracking. A representative transvarestraint sam­ple is shown in Figure 6, which illustrates the locations of solidification — and liquation-type cracks. Note that solid-state cracks can also be produced in this test.10

In general, alloying additions that are rejected into the liquid (i. e., whose equilibrium segregation coeffi­cient, k, is <1) lower the solidus and increase the solidification temperature range. This increases the extent of the solid + liquid ‘mushy’ zone and increases the susceptibility to solidification cracking. For exam­ple, niobium and molybdenum have segregation

(a)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Fraction solid

(b)

5 mm

5 mm

Mo L|

(c)

Figure 4 Illustration of the effect of solidification path on cracking resistance. (a) Two Ni-30Cr filler metals show markedly different hot crack susceptibilities. (b) Scheil modeling predicts that the more solute-rich alloy has a larger solidification temperature range and can form a, Laves and S in the terminal solid and (c) SEM investigation confirms Nb, Mo-rich Laves near solidification cracks.

coefficients <1 in austenitic stainless steels and nickel — based alloys, which explains the longer crack lengths in 347 SS than in 308L SS (see Figures 5 and 7). Similarly, in nickel alloys, the susceptibility to solidification
cracking is Alloy 625 > EN52MSS > EN52i > EN82H > 68HP, which is a direct result of the decreasing alloying content ofthe strong melting point depressants molybdenum and niobium.7,10 15