What isTemper Bead Welding (TBW) or Controlled Weld Deposition (CWD)
Welding repair on aged and service exposed pressure equipment and piping and other components are always a notorious task for welding engineers/ specialists. As we know that Material properties get degraded when they are exposed to elevated temperature services e.g. in petrochemical, power plants & steel plants. Due to inferior and degraded mechanical properties, welding of these materials is a very challenging task for the welding engineers because:
a. Microstructure prone to change to martensite
b. Controlling the weld & HAZ hardness
c. Minimizing the residual stresses
d. controlling the grain refinement
e. Microstructure detrimental to toughness properties
The option to regain the mechanical properties such as strength, ductility, and toughness calls for the post-weld heat treatment. But full PWHT is not possible in in-service plants, due to the size of equipment (Large size parts), the extent of the repair and material properties, material property issues after many years of service-related exposures, and many other constraints to names few reasons.
Definition of temper bead welding:
ASME Definition: ”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 (HAZ) or previously deposited weld metal. The bead may be above, flush with, or below the surrounding base metal surface. If above the base metal surface, the beads may cover all or only part of the weld deposit and may or may not be removed following welding.”
In simple words, TBW is used as a welding technique, when carbon steel, low alloy steel, or alloy steel is designed to be used to be in as-welded condition or a PWHT is impractical.
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Introduction to Temper Bead Welding
Temper bead welding technique: Originally developed as an alternative to Post Weld Heat Treatment (PWHT) welds mandated by design and construction codes. Temper bead welding offers benefits as listed below:
- Help to reduce as-welded HAZ hardness.
- Beneficial for repair welding for large structures for which it is difficult to perform the specified post weld heat treatment.
- To refine the coarse grained HAZ in the parent metal.
- Help to reduce residual stresses
- Provide localize PWHT of previous beads
Precaution: TBW technique is not a substitute for post weld heat treatment (PWHT) in conditions where service conditions as applicable with code requirements require a considerable reduction in residual stress levels inter alia to other requirements. This heat treatment aims to improve locally the fracture toughness and reduce the peak hardness within the HAZ produced by the weld repair or in-service weld.
Governing Codes and Standards
National Bard Inspection Code (NBIC) is the most common code followed for repair and alteration by non-nuclear industries. In NBIC, certain conditions such as post weld heat treatment, in accordance with the original code of construction, may be inadvisable or impractical. Alternative methods for PWHT may be used in such instances. One of the recommended alternatives is Temper Bead Welding Technique. NBIC Part 3, Clauses 188.8.131.52 thru 184.108.40.206 are methods in which the welding procedure requires the use of a temper-bead welding technique.
ASME Sec-III, NB-4622.9 allows limited weld repairs on P-No. 1 and P-No. 3 material, without PWHT or after the final PWHT, provided it is impossible or impractical to post-weld heat treat the area after repair, and provided further requirements according to this code are met.
Metallurgical Principles (Microstructural Changes) Behind Weld & HAZ
Fig-1 shows the various microstructural changes occurring during a typical arc welding process. This helps to understand what takes place in the HAZ due to welding and post-weld heat treatment, or temper bead welding. The reference here is temperature ranges vs the resultant microstructure with respect to the iron-carbon phase diagram. These microstructural changes are valid for many grades of conventional Ferritic Steels (Carbon & Creep Resisting) used in the construction of power /petrochemical & refinery plants.
Temper Bead Welding Techniques
During temper bead welding, the heat input, preheat and weld bead sequence are closely controlled to:
- a) Limit heat input and pre-heat to avoid excessive grain coarsening of the ‘coarse grained’ HAZ of the first weld layer;
- b) Increase heat input by a set amount for the second weld layer to grain refine the coarse grained HAZ of the underlying first weld layer;
- c) Overlap the placement of successive weld beads to produce grain refinement of the adjacent bead.
Multiple temper bead welding techniques have been developed worldwide by various research Centre and industries. Below are the important techniques used:
- a) Half Bead Technique
- b) Controlled Weld Deposition Technique
Qualification of Temper bead Welding Procedure
Temper-bead welding procedure qualification nomenclature is defined in Section IX of the ASME Boiler and Pressure Vessel Code. Typically, this technique minimizes heat input of the initial beads, thus limiting heat beyond the weld heat-affected zone (HAZ) of the base metal. Heat input shall be increased for successive beads in accordance with the rules of QW-290 for temper bead welding in ASME Section IX.
The six sub-clauses under this section provide the requirements for procedure qualification requirements, restrictions, essential and non- essential variables, test coupon preparation and testing and in-process repair welding. Additionally, sub-clause QW-290.6 provides supplementary qualification requirements for the welders who will be carrying out the work.
ASME Sec-IX is most commonly followed code for welding procedure and performance qualification. QW-290 addresses all the relevant clauses for Temper bead welding. When the applicable Code Section specifies the use of this paragraph for temper bead welding, QW-2 90.1 through QW-290.6 shall apply. The appropriate clauses are as follows:-
- QW-290.1 Basic Qualification and Upgrading Existing WPSs
- QW-290.2 Welding Process Restrictions
- QW-290.3 Variables for Temper Bead Welding Qualifications
- Table QW-290.4 Welding Variables for Temper Bead Procedure Qualification
- QW-290.5 Test Coupon Preparation and Testing
- QW-290.6 In-Process Repair Welding
The test specimen layout as shown in Sec-III, NB is extracted under Fig-3 below.
In order to assess the metallurgical effectiveness of the temper bead welding procedure on the materials being repaired required the following four supplementary tests are generally applied:
1) Microstructural assessment of Weld and HAZ
2) Hardness Testing
3) Charpy Vee Notch Impact Testing
4) Bend Test
In order to determine the effectiveness of the temper bead welding procedure, it will be necessary to compare the temper bead HAZ results with:
The same results obtained from the conventional welding procedure qualification test after the traditional post weld heat treatment;
- b) The same test results obtained from the conventional welding procedure without post weld heat treatment.
Both comparisons are required to determine if the applied temper bead welding technique provides the required results.