Stress-Relieving Treatments

Weld Stress and Treatment

Stress in a weld can cause poor performance and cracking in service. Weld stress is caused by a number of factors, including the following:

• The hardening of the weld's heat affected zone
• Contraction of the cooling weld
• Hydrogen in the base metals

Stress-relieving treatments prepare the base metals for welding and temper the heat affected zone after welding. These treatments include the following:

• Preheating
• Hydrogen bakeout
• Post-weld heat treating

Preheating

his section describes the preheating Function, Process, and Guidelines.

Function

Preheating performs three basic functions:

• Primarily, it slows the cooling rate of the HAZ.
• It minimizes martensite formation in the weld and HAZ.
• It ensures that the steel surface is dry before welding.

Process

Preheating requires both heating and testing. Preheating is performed using two methods:

• Electric resistance with thermocouples and insulation
• Torch heating

Preheating slows the cooling rate and minimizes martensite formation. Welders use electric resistance or torch heating to heat the base metals to the desired temperature. To ensure that the preheating requirements are being met, welders use melt sticks to check the steel temperature a short distance from the weld.

To dry steel surfaces, welders use a torch to preheat the steel above 200°F. Preheating eliminates moisture which can cause weld defects and lead to HAZ cracking.

Guidelines

Two factors must be considered when establishing preheating guidelines: Desired Microstructure and Steel Thickness.

Desired Microstructure

The microstructures formed in the steel after welding are a function of the preheating temperature and cooling time. Higher preheat temperatures produce softer microstructures. Figure 20 shows the effects of preheating on CCT curves.

Figure 20: Effects of Preheating on CCT Curves

Table 14 identifies the microstructures formed at different preheating temperatures.

Table 14: Effects of Preheating on CCT Curves

For an explanation of the physical properties of each microstructure, see Weld Metallurgy.

Many refinery applications require that the weld HAZ hardness be below 200 Brinell to be acceptable for service and to avoid environmental cracking. Only the weld made with the 500°F preheat (curve 3) would be soft enough in the as-welded condition to avoid cracking. Sometimes a 500°F preheat is not practical or field conditions prohibit it. The welder then chooses one of two options:

• A post-weld heat treatment only (with no preheat)
• A standard (200°F) preheat plus a post-weld heat treatment

Preheating reduces the formation of martensite in the weld. A post-weld heat treatment (PWHT) performed after welding eliminates residual stress in the weld.

Steel Thickness

Steel thickness is also a key factor in determining preheat guidelines. Preheating is essential for thick steel but unnecessary for thin. The base metal of thick steels welded without preheating absorbs the heat rapidly and the weld cools rapidly. For thin sections, the heat of welding sufficiently warms the surrounding steel, slows cooling rates, and minimizes the formation of martensite. Many welding codes contain preheating guidance based on preheat-thickness relationships.

Hydrogen Bakeout

This sections describes a hydrogen bakeout's Function, Process, and Guidelines.

Function

A hydrogen bakeout is a special preheating method used to remove hydrogen absorbed by the steel before welding. Hydrogen bakeouts are used before making weld repairs on process units with high partial pressures of hydrogen. Hydrogen-charged environments include process streams containing liquid water with H₂S, amines, or cyanides. Hydrogen absorbed in the metal of these units can cause blisters or hydrogen cracks. Before the metal can be welded, the hydrogen must be removed, or "baked out." Hydrogen left in the steel will "bubble out" in the weld bead, causing a defective weld.

Process

A hydrogen bakeout involves a two-step process:

1. Furnaces or electric resistance equipment heats the steel to 500-700°F and holds it at that temperature for several hours to allow the hydrogen to diffuse from the steel.
2. The temperature is then dropped to the preheat temperature in preparation for welding.

Guidelines

The effectiveness of a hydrogen bakeout depends upon three factors:

• Steel thickness
• Bakeout temperatures
• Bakeout time

Thick steel requires longer bakeout times and sometimes higher bakeout temperatures.

Post-Weld Heat Treating

This section describes post-weld heat treating's Function, Process, and Guidelines.

Function

Post-weld heat treatment (PWHT) uses heat to temper the hard steel phases (martensite and bainite) formed during welding. This tempering serves two functions:

• It softens and toughens the weld.
• It lowers the level of residual stress, making the steel more resistant to the following problems:
  • Distortion or cracking under applied load
  • Fatigue cracking under cyclic loads

PWHT temperatures are too low to change the microstructure or cooling curve of the steel.

Process

PWHT involves two basic steps:

1. Furnaces or electric resistance equipment heats welded sections to the specified temperature--up to but not exceeding the austenitizing temperature for that grade of steel (1342°F for carbon steel).
2. This temperature is maintained for the specified time.

Guidelines

Changes in PWHT time and temperature produce different levels of microstructural softening. Raising PWHT temperatures produces the same effect as lengthening PWHT time. In other words, the higher the PWHT temperature, the shorter the PWHT time. Short PWHT cycles at higher temperatures are usually more cost-effective than longer cycles at lower temperatures.

Table 15 shows the relationship of PWHT temperature on microstructural softening.

Table 15: The Effect of a 1-Hour PWHT on the Vickers Hardness of 1-1/4 Cr, 1/2 Mo Steel

PWHT has some limitations. For example, PWHT should not be used on the following:

• Vessels with severe blistering problems
• Some dissimilar metal joints involving austenitic stainless steels

To make determining the PWHT requirements easier, piping codes and Amoco specifications specifically detail the PWHT requirements for the most commonly used steels.

For more information on the treatment of austenitic stainless steels, see Special Welds.