Temper Bead Welding (TBW)/ Controlled Weld Deposition (CWD)

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In the heavy fabrication and repair industry, Post Weld Heat Treatment (PWHT) is the standard method for reducing residual stress and softening brittle Heat Affected Zones (HAZ). But what happens when you need to repair a steam drum inside a power plant? Or a pipeline that is still in service? You can’t exactly put a 50-ton vessel into a furnace, and local PWHT might damage internal components or warp the structure.

Solution lies with Temper Bead Welding (TBW), also known as Controlled Weld Deposition (CWD).

This is the “Special Forces” technique of the welding world. It is a precise, engineered method of bead placement that uses the heat of the welding arc itself to perform the heat treatment. When done correctly per ASME Section IX QW-290, it eliminates the need for furnace stress relief, saving millions in downtime.

This guide breaks down the metallurgy, the technique, and the critical code requirements you need to know.

What is temper Bead Welding (TBW) or Controlled Weld Deposition (CWD)

Performing welding repair on materials that are exposed to high temperatures is a challenging job. Most of the materials (Low alloy steel e.g. Cr-Mo types, Cr-Ni Alloys e.g. SS347, Incoloy 800, etc.), when exposed to high temperature during service, will result in aging and brittle microstructure due to various metallurgical changes.

Example applications of these materials are in power plants, refineries, and other related areas.

As we know Material properties get degraded when they are exposed to elevated temperature services e.g. in petrochemical, power plants & steel plants static equipment & piping. Due to the resultant inferior and degraded mechanical properties, performing welding of these materials is a very challenging task for the welding engineers because:

• Microstructure prone to change to martensite during welding repair.
• Controlling the weld & HAZ hardness.
• Minimizing the residual stresses.
• Controlling the grain refinement.
• Microstructure detrimental to toughness properties.

The best option to regain the mechanical properties such as strength, ductility, and toughness calls for the post-weld heat treatment (PWHT). But full PWHT is not possible due to in-service equipment operation, due to the size of equipment (Large size parts), the extent of the repair and material properties, etc.

So, it is always not an on-hand offer to make the heat treatment to bring back the material to its normal microstructure or stress conditions.

The Metallurgy: How it Works

To understand TBW, you must understand the Heat Affected Zone (HAZ).

When you deposit a weld on ferritic steel (like Carbon or Chrome-Moly), the base metal adjacent to the weld is heated rapidly and cooled rapidly (quenched). This transforms the steel into Martensite—a hard, brittle microstructure that is prone to cracking. Usually, we put the part in a furnace ($600^{\circ}\text{C}$) to “temper” this martensite, allowing the carbon to diffuse and the metal to become tough again.

In Temper Bead Welding, the “Second Pass” acts as the furnace.

1. Layer 1 (Butter Pass): Creates a hard, brittle HAZ in the base metal.
2. Layer 2 (Temper Pass): Is deposited over Layer 1. The heat from Layer 2 penetrates through Layer 1 and into the original HAZ.
3. The Result: If the heat input is perfectly controlled, Layer 2 heats the original HAZ to a temperature below the critical transformation point ($A_1$) but high enough to temper the martensite.

Definition of temper bead welding:

The definition of Temper Bead Welding is clearly stated in BPVC ASME Section IX Code. ASME Section IX, QG 109-Definitions, stated the Temper bead Welding as:

”a weld bead placed at a specific location in or at the surface of a weld for the purpose of affecting the metallurgical properties of the heat‐affected zone or previously deposited weld metal.’‘ 

Reference: ASME Section IX, QG 109 2025 Edition.

In simple words, this welding method is used, for many materials such as CS material, Cr-Mo alloys, and alloy steel when a PWHT is impractical due to service conditions or other limitations.

TBW helps to achieve various objectives by a pre-planned approach by:

1. Placing weld beads at pre-determined locations.
2. Tempering the base metal, preceding weld beads/ weld passes.
3. Relieving the residual stresses.
4. Controlling the weld cooling rates to eliminate hard microstructures.

Watch our YouTube video for interactive animation learning on Temper Bead Welding

The Techniques: Old vs. New

A. The “Half-Bead” Technique (Legacy)

This was the original method used in the nuclear industry (ASME Section III).

1. Weld the first layer with small electrodes (e.g., 2.4mm / 3/32″).
2. Grind off exactly 50% of the first layer.
3. Weld the second layer with larger electrodes (e.g., 3.2mm / 1/8″).

• The Logic: Grinding brings the second layer closer to the HAZ, ensuring the tempering heat reaches the target zone.
• The Problem: It is incredibly labor-intensive and relies on the grinder’s skill to remove exactly 50%.

B. The “Consistent Layer” Technique (Modern)

This is the standard today (ASME Section IX / API 510).

• No grinding required.
• Controls are based strictly on Heat Input ratios and Bead Overlap.
• The welder uses specific amperages and travel speeds to ensure Layer 2 provides the exact thermal profile needed to temper Layer 1.

Code Compliance: ASME Section IX (QW-290)

Before 2004, Temper Bead rules were scattered. Now, Article IV, QW-290 is the “Bible” for this process.

The Qualification (PQR)

Qualifying a TBW procedure is much harder than a standard PQR.

1. Hardness Testing: This is the ultimate pass/fail. You must take Vickers or Rockwell hardness surveys across the weld, HAZ, and base metal.
   • Typical Limit: Max 248 HV10 (for Carbon Steel) or roughly 22 HRC.
2. Impact Testing: Charpy V-Notch tests are usually mandatory to prove toughness.
3. Microscopy: Sometimes required to visually verify the grain refinement.

Essential Variables (The “Killers”)

If you change these, you must requalify:

• Heat Input Ratio: You cannot simply “weld hot.” You must maintain a specific ratio between the heat input of the first layer and the second layer.
• Bead Overlap: Usually 50% overlap is mandated. If the welder changes to 20% overlap, the tempering effect is lost.
• Electrode Size: You cannot change rod diameters indiscriminately.
• Preheat & Interpass Temp: These ranges are extremely tight (e.g., 150°C ± 25°C).

Objectives of Temper Bead Welding

In temper bead welding, the heat input of successive layers temper previous beads, resulting in a fine grain structure and low hardness in the heat affected zone (HAZ).

This process is often used for welds that need to avoid post-weld heat treatment (PWHT). It can because of impractical PWHT situations such as welding repair in a in-service plant. Temper bead welding can produce welds with good mechanical properties and a small HAZ.

Temper Bead Welding’s main objective is to eliminate the need for post-weld heat treatment (PWHT) after weld completion where is mandatory to perform the PWHT due to material type, code requirement, or client specification.

In general, Temper bead welding serves to achieve the following objectives:

1. Help to reduce as-welded HAZ hardness.
2. Beneficial for repair welding for large structures for which it is difficult to perform the specified post-weld heat treatment.
3. To refine the coarse-grained HAZ in the parent metal.
4. Help to reduce residual stresses.
5. Provide localize PWHT of previous weld beads. 

Temper Bead Techniques

There are a variety of temper bead techniques that can be used to achieve different results. Here are five of the most popular:

a) Half Bead Technique
b) Consistent Layer Technique
c) Alternate Temper Bead Technique
d) Controlled Deposition Technique
e) Weld Toe Tempering Technique

Qualification of Temper Bead Welding Procedures

Welding Procedure Specifications (WPS) and Procedure Qualification Records (PQR) are the foundation of a sound welding quality program.

In order to ensure that all qualified WPS results in required weldment properties, it is necessary to follow the requirements set forth in ASME Section IX. This section covers all details for WPS-PQR, including welder qualification, welding operator qualification, and production welding for Temper Bead welding qualification.

The first step in qualifying a temper bead welding procedure is to establish the base metal specifications. The base metal thickness, grade, and heat treatment must all be taken into account when determining the appropriate filler metal and weld parameters.

Once the base metal specifications have been established, the next step is to qualify the welder. The welder must be qualified in accordance with ASME Section IX before they can begin welding on production parts to carry out the temper bead welding.

Test Required for Temper Bead Procedure Qualification

A test is required for temper bead procedure qualification in order to assess the microstructure of the weld and heat-affected zone (HAZ). The hardness and CVN values are also important factors in determining the success of the temper bead procedure. The test results will help determine if the weld and HAZ can meet the required strength and toughness properties.

The bend test is used to determine the ductility of the weld metal. The Charpy V-Notch is used to determine the toughness of the weld metal.

A Summary of test generally required for Temper Bead Procedure Qualification are mainly as discussed above:

• Microstructural Test: For weld metal, HAZ and adjacent base metal
• Hardness Test: For weld metal, HAZ and adjacent base metal
• Charpy V-Notch Test
• Bend Test

Step-by-Step Execution in the Field

Step 1: Surface Preparation

The base metal must be chemically clean. Any hydrogen sources (rust, oil, paint) will cause underbead cracking, which TBW is specifically trying to prevent.

Step 2: Preheat & Hydrogen Bake-Out

• Preheat: Usually higher than normal (150°C- 200°C) to slow the cooling rate.
• De-Hydrogenation: Often, the area is heated to 300°C for 2-4 hours before welding to drive out trapped hydrogen.

Step 3: Buttering (The Pad)

The welder deposits a “butter layer” using specific run-out lengths.

• Technique: Stringer beads only. No weaving. Weaving creates uncontrolled heat input.
• Sequence: The beads are often laid in a specific pattern (e.g., from the center out) to manage stress.

Step 4: The Temper Passes

The subsequent layers are applied. The welder must aim for the “toe” of the previous bead to ensure 50% overlap.

• Stop/Start Locations: Staggered. Never start a weld on top of a start/stop from the layer below.

Step 5: Post-Weld Soak (DHT)

While technically “Exempt from PWHT,” most TBW procedures require a De-Hydrogenation Heat Treatment (DHT).

• Immediately after welding, raise temp to 250°C- 300°C and hold for 2-4 hours.
• This allows any hydrogen introduced during welding to diffuse out before the steel cools and cracks.

When to Use (and When to Run Away)

✅ Use TBW When:

• In-Service Repair: You are welding a patch on a pipeline that contains product.
• Massive Structures: Repairing a Hydrocracker or Nuclear Reactor vessel where field PWHT is physically impossible.
• Distortion Concerns: When PWHT would warp machined surfaces (e.g., turbine casings).

❌ Do NOT Use TBW When:

• Creep Service: If the component operates in the creep range (high temp), TBW might not provide the microstructural stability needed for long-term life.
• Stress Corrosion Cracking (SCC): TBW softens the HAZ, but it leaves Residual Stress. If the service fluid causes SCC (e.g., Caustic, Amine), TBW is dangerous because the residual tensile stresses are still present. You need thermal PWHT to remove stress.

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