What are various Temper Bead Welding Techniques?
When performing a temper bead welding, welding heat input, preheat & interpass temperature and placement of weld bead required to be closely monitored and controlled to:
- a) Limit heat input and pre-heat to avoid excessive grain coarsening of the “coarse-grained” Heat affected zone (HAZ) of the first weld layer made on the component.
- b) For the 2nd weld layer, Increase the heat input by a set amount (usually higher heat input than the previous bead) to refine the coarse-grained HAZ of the underlying first weld layer due to the tempering effect produced by the thermal heat of 2nd weld layer.
- c) Overlap the placement of successive weld beads to produce grain refinement of the adjacent weld beads.
Types of Temper Bead Welding Techniques
Based on numerous works carried out in last two decades, five temper bead welding techniques have been developed as given below:
1) Half Bead Technique
2) Consistent Layer Technique
3) Alternate Temper Bead Technique
4) Controlled Deposition Technique
5) Weld Toe Tempering Technique
All of the above techniques have the same goal of tempering the coarse-grained HAZ in the parent metal & the previous weld beads/ layers. The methods by which the weld beads are deposited vary from technique to technique.
Half Bead temper welding Technique
The technique was initially established for use in the nuclear industry but has since become widely used for repairs to piping, headers and turbine casings in conventional power plant.
The SMAW technique is used and employs a series of increasing diameter electrodes, starting with 2.5 mm, then 3.2 mm, and finishing with 4.0 mm electrodes. The increasing diameters provide a sufficient increase in heat input from the first to the third layer.
The area to be repaired is cleaned and preheated to a temperature corresponding with the material and thickness (typically >150° C), and a buttering technique used as the first layer with 2.5 mm electrodes. The use of 2.5 mm electrodes is to produce a small, shallow heat-affected zone.
The second step is to remove approximately half of the welded layer by grinding.
The third step is the deposition of a second layer using 3.2 mm electrodes. This effectively re-transforms the coarse-grained heat affected zone and first layer.
The fourth step is again to remove approximately half of the welded layer by grinding.
The remaining steps are the deposition of third and subsequent layers using 4.0 mm electrodes with further grinding after each layer has been deposited. Each subsequent layer transforms and tempers the layers beneath it.
The disadvantage of the technique is that a lot of accurate grinding is required. This is time-consuming and if too much material is removed from the first layer, the effects of the re- transformation are not successful. As a result, this technique has now lost favor.
Consistent Layer temper welding Technique
This technique utilizes either the SMAW or the GTAW process. The technique involves depositing weld layers that are sufficiently thick that the subsequent weld layer only tempers the heat-affected zone caused by the first layer. The temperature is not intended to exceed the lower crystallization temperature “A1” (Click here to calculate lower & upper re-crystallization temperature for any steel), so no grain transformation occurs. The technique can produce a heat-affected zone microstructure that consists predominantly of tempered martensite with small amounts of bainite, resulting in good toughness properties.
Alternate Temper Bead Welding Technique
This technique was developed specifically for carbon-manganese and carbon-molybdenum materials used in nuclear reactor pressure vessel components. It utilizes the automatic GTAW process and is an alternative to the half-bead technique for use in areas of high radiation exposure.
The technique involves preparing the area to be repaired so that at least six buttering layers can be performed. A preheat of 1500° C minimum is applied, and the heat input of each layer is controlled to within 10% of that measured in the procedure qualification test.
Controlled Deposition temper welding Technique
This technique resulted from special cases where creep embrittlement and re-heat cracking were potential problems during repair and were aimed at specific materials used in conventional fossil-fired stations. It is also a SMAW technique and uses strictly controlled ratios of heat input between one weld layer and the next. The heat input for the second layer is 1.3 to 1.8 times higher than for the first layer and is designed to produce grain refinement and tempering in the original heat-affected zone. The ratios need to be experimentally verified for each material to be welded.
The increase in heat input should be 30 to 70% for each subsequent layer, and for SMAW, increasing the electrode size by one size whilst keeping the welding technique the same generally achieves this.
It is not necessary for production to use exactly the same heat input as in the procedure test, but the ratio between layers must remain the same.
Part of the weld metal and HAZ of the first bead is re-melted by the second bead. How much is re-melted depends on the overlap, but typically, a 50% overlap is the aim. The temper bead technique usually involves completing the first layer first.
The heat input for the second layer runs is increased so that the heat re-melts some of the first layers but re-transforms the coarse-grained area of the HAZ while tempering the inter-critical region. Extreme care is needed with the placement of the final run. The third layer is a repeat of the second layer with increased heat input.
Weld Toe Tempering Technique
This technique involves the placement of a sacrificial capping using beads, runs or weld sequence to temper the underlying HAZ at the toe of butt or fillet welds. The bead placement sequence is crucial to achieving the desired effect. The temper capping beads are usually ground off to bring the weld reinforcement back to acceptable limits. Whilst the virtues of temper bead welding techniques are debatable, there is no doubt that the HAZ at the weld toe is one of the hardest areas when left in the as-welded condition. The weld toe tempering technique provides a simple method to temper this area without resorting to PWHT. It has been applied for many years.
QUALIFICATION OF TEMPER BEAD WELDING PROCEDURES
ASME Section IX has provided full details on temper bead qualification testing in accordance with the ASME code. The qualification process for temper bead welding must establish that the welding technique produces the required heat treatment of the HAZ in addition to meeting the traditional welding procedure qualification requirements.
To determine if the temper bead welding procedure produces the required HAZ the following four supplementary tests are generally applied:
1). Microstructural assessment of Weld and HAZ This assessment will determine the presence of un-tempered martensite, regions of excessive grain growth and the presence of any undesirable microstructures.
2). Hardness Testing A systematic hardness survey across the weld and HAZ will provide rapid evidence of undesirable hard microstructures. A micro-hardness testing survey across the HAZ using Vickers or Knoop hardness testing methods provides a good indication of whether the desired microstructures in the HAZ were achieved.
3). Charpy Vee Notch Impact Testing Charpy Vee Notch testing of the as-welded HAZ provides a well-recognized determination of the notch toughness of the HAZ.
4). Bend Test The traditional side bend test provides a rapid and inexpensive method of determining the ductility and strain capacity of the HAZ.
In order to determine the effectiveness of the temper bead welding procedure it will be necessary to compare the temper bead HAZ results with:
- a) The same results obtained from the conventional welding procedure qualification test after the traditional postweld heat treatment;
- b) The same test results obtained from the conventional welding procedure without postweld heat treatment.
Both comparisons are required to determine if the applied temper bead welding technique provides the required results.