The ABC's of Arc Welding

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A cold crack is generally referred to as a spontaneouscrack that occurs at temperatures blow 200℃ aftersolidification is complete in welding. Cold cracking candevelop hours or days after the weld has been made,which is referred to as "delayed cracking." Coldcracking is likely to occur in all ferritic and martensiticsteels such as carbon steel, low alloy steel and high alloysteel unless adequate precautions, mainly preheating, areemployed.

Cold cracks, as shown in Photos. 1 and 2, are causedby the combined effects of low ductility of the weld,residual stress and diffusible hydrogen in the weld. Aweld's ductility may decrease with a high carbonequivalent and a high cooling speed after solidification.Residual stress in a weld can be larger than expected if itcontains weld discontinuities such as incomplete fusion,incomplete joint penetration, overlap, undercut, slaginclusions, and porosity. The source of diffusiblehydrogen in a weld is mainly moisture in the weldingconsumable and atmosphere.

Photo 1. A cold crack initiated at theroot of the weld anddeveloped into the weldmetal

Table 1. A general guide to the preheating and interpasstemperatures for various types of steels

Type of metalPreheating�and interpass�temperature (℃)

HT 50 20 - 120HT 60 50 - 120HT 80 100 - 1800.5%Mo steel 100 - 2001.25%Cr-0.5%Mo steel 150 - 3002.25%Cr-1%Mo steel 200 - 3505%Cr-0.5%Mo steel 250 - 3509%Cr-1%Mo steel 250 - 35017%Cr stainless steel 100 - 25013%Cr stainless steel 100 - 400Note: �These temperatures are for general reference only.

Photo 2.A cold crack initiated at the toeof the weld and developed into

the heat-affected zone

In welding heavy-thick pressure vessels, immediatepostheating (e.g. 300℃×30min) is often applied on theweldment right after welding finished, while the weldmaintains the preheat and interpass temperature. This isto ensure no delayed cracking when the vessel is cooledto room temperature for nondestructive examinationsbefore postweld heat treatment (PWHT). The combineduse of preheating and immediate postheating is effectiveto prevent cold cracks in the weld.

Cold cracking, therefore, can be prevented bycontrolling the three main factors－low ductility,residual stress, and diffusible hydrogen. That is,

(1) Preheat the base metal to reduce the cooling speed ofthe weld. This prevents the embrittlement of theweld and removes dissolved hydrogen from theweld.

Cold Cracks: Causes andCures

(2) Prevent weld discontinuities to avoid stressconcentration.

(3) Use low-hydrogen type welding consumables tominimize diffusible hydrogen in the weld.

On preheating, it is important to determine thetemperature appropriate to the base metal and fillermetal to be used. The appropriate temperature isgenerally determined for individual work by taking intoaccount several factors such as chemical composition,restraint level (or plate thickness), the welding process,heat input, and the amount of diffusible hydrogen in theweld metal. Table 1 is a quick guidance to the properpreheating temperatures for different types of steels.

Redrying covered �electrodes is �effective to prevent �cold cracks.��