Avoid postweld heat treatment - use fracture mechanics

TWI Bulletin, November - December 1996

Alan Smith is a Senior Project Engineer working in the defect assessment section of the Structural Integrity Department at TWI. Since joining TWI, he has been actively involved in residual stress measurement and fitness-for-purpose assessments of welded structures.

Fracture mechanics analysis is widely used in offshore applications to justify exemption from postweld heat treatment (PWHT). Alan Smith describes the background to this development, and gives a worked example illustrating the application of a fracture mechanics analysis to a girth weld in a tubular structure.

In many applications, the reason for applying PWHT is to avoid the risk of brittle fracture. This is achieved by lowering the level of residual stress in the structure and (in some applications) by increasing the fracture toughness of the fracture sensitive region.

Current guidelines for application of PWHT, such as HSE offshore guidance and BS 5500: 1994 are based on a thickness criterion. This does not explicitly recognise any influence from fundamental parameters which control fracture of the welded joints: namely, stress, toughness and defect geometry.

An alternative method for deciding whether to select PWHT is to consider the effect of PWHT on fracture using a fracture mechanics based model. This procedure is given in PD6493: 1991, ^[1] and is recognised in HSE offshore guidance and BS 5500. Although fracture mechanics is widely used in the offshore industry, there remains considerable scope for increased use of fracture mechanics to avoid PWHT in other applications.

This paper describes the advantages of using a fracture mechanics based procedure compared to a thickness based criterion. It illustrates how PD6493 can be used to determine whether PWHT is necessary for avoidance of failure by brittle fracture. However, in cases where exemption from PWHT is sought, consideration should be given to the influence of heat treatment on all relevant failure mechanisms.

Background

Recommendations for PWHT of offshore structures are given in Offshore Installations Guidance on Design, Construction and Certification. ^[2] HSE guidance states PWHT is normally required for:

1. Welds in plated and tubular nodal joints for which the 'hot spot' stress exceeds 80% of the specified minimum yield stress of the parent plate and if the thickness of the thinnest plate exceeds 40mm,
2. Regions other than those defined above, if the thickness of the thinnest plate being joined exceeds 50mm.

However, the guidance recommends that PWHT may be omitted, where satisfactory performance in the as-welded condition can be demonstrated by both Charpy and fracture toughness tests on samples notched in both weld metal and heat affected zone (HAZ) from procedure test plates.

Where exemption is sought from PWHT, a fracture mechanics analysis is required to evaluate the significance of defects found in fabrication or during service. Regulations concerning the decision on whether to use PWHT are different depending on the Certifying Authority and can be found in detail elsewhere. ^[3-5]

Requirements for PWHT pressure vessels are given in 'Specification for Unfired Fusion Welded Pressure Vessels' BS 5500: 1994, ^[6] which recommends PWHT in the following cases:

• ferritic steel vessels designed to operate above 0°C where the thickness at any welded connection exceeds that listed in BS 5500 Table 4.4.3 unless otherwise agreed between purchaser, manufacturer and Inspecting Authority to permit a greater thickness based upon fracture mechanics analyses in accordance with Appendix U.
• ferritic steel vessels designed to operate below 0°C when PWHT is necessary in accordance with Appendix D.

where both Appendix D and Appendix U direct the user to the option of a PD6493 type assessment, using fracture toughness data obtained in accordance with BS 5447 or BS 5762.

 Basis for the thickness requirement for offshore structural steels is given in the background document. ^[7] An extensive experimental programme was conducted on welds and HAZ, representative of the steel used in offshore construction during the 1970s, Fig.1. A review of the results concluded that for weld metal exceeding 40mm in thickness the fracture toughness was limited at temperatures below -10°C in the as-welded condition. Heat affected zone fracture toughness was not a cause for concern and acceptable fracture toughness values could be achieved without PWHT. It was also shown that the Charpy test could not discriminate between weld metals with poor and acceptable toughness in the as-welded condition. Consequently PWHT was recommended for highly stressed areas where the thickness exceeded 40mm, and 50mm elsewhere. The 40mm thickness criterion was consistent with established pressure vessel practice. ^[6]

More recently the effect of PWHT on the 27J Charpy transition temperature of submerged arc (SAW) and manual metal arc (MMA) has been reviewed by Pisarski, ^[8] who concluded that PWHT can have either a beneficial or adverse effect on the Charpy toughness of weld metal. It follows that PWHT should not be automatically assumed to improve fracture toughness of the weld metal. However, regardless of the effect of PWHT on fracture toughness, PWHT will lead to improved resistance to brittle fracture through a lowering of the residual stress. In PD6493 it is assumed that PWHT leads to a reduction of residual stress to 15% of the weld metal yield stress for stresses transverse to the weld, and to 30% of the weld metal yield stress for stresses parallel to the weld. These values of the PWHT residual stress are dependent on both the PWHT temperature and dwell time. The levels of residual stress could be significantly higher at PWHT temperatures of less than 550°C.

In view of conflicting evidence for and against improvement of the fracture toughness properties with PWHT compared to the as-welded condition, use of a criterion based on thickness appears to be less than satisfactory. In some cases the thickness criterion could lead to use of PWHT where it is not necessary. Use of fracture mechanics testing and analysis as recommended in the Guidance Notes should rationalise the choice of PWHT and could lead to substantial cost reductions, or alternatively, safer operation. The same logic can be applied to both the offshore and pressure vessel industries.

PD6493 to determine exemption from PWHT

BS PD6493 considers the significance of weld flaws using a fitness-for-purpose approach. Risk of brittle fracture is assessed by determining the linear elastic solution for the stress intensify factor (SIF) and comparing it to the critical fracture toughness of the material. Risk of local plastic collapse is assessed by comparing the net section stress to the flow strength. Three levels of fracture assessment are considered: an initial screening method; an intermediate 'normal' assessment and an advanced assessment which reviews strain hardening behaviour. In the following case studies the intermediate, Level 2 assessment is used. This accounts for non-elastic behaviour using a strip yield model (described in more detail in PD6493: 1991).

The Level 2 approach can be used to determine:

1. if a flaw of known size is acceptable under prescribed loading conditions,
2. the maximum tolerable flaw dimensions which the structure will withstand.

The approach taken here is to define the material properties (including yield strength, tensile strength and fracture toughness) and applied stresses, and then to determine maximum tolerable flaw sizes with and without PWHT. These can then be compared to NDT inspection reports to determine whether the tolerable flaw is detectable. Alternatively, one could start from the assumption of a known flaw size and determine either the minimum fracture toughness, or maximum safe applied stress, with and without PWHT. The approach adopted in this study is shown in Fig.2. For fatigue sensitive regions the maximum tolerable initial flaw size would be determined, and this flaw size compared to NDT inspection records.

Example

PWHT of girth weld in a tube subject to a tensile load

The following example is designed to illustrate application of a fracture mechanics procedure to a girth weld in a tubular structure. The example chosen is simple. The procedure can be applied to more complex constructions provided that suitable expressions are available for the SIF and the net section stress, and suitable input data are available.

Integrity of the welded joint should be assessed in terms of all potential defects. This example concentrates on the effect of thickness on the limiting tolerable flaw size for a surface crack parallel to the girth weld.

Input assumptions

• long surface crack (a/c →0) analogous to a lack of fusion defect in a girth-welded tube of 0.8m diameter with varying wall thickness of 15-75mm.
• crack is located at the toe of a butt weld with a weld toe angle of 45°C and weld bead width equivalent to 1.16 of the plate thickness.
• applied stress = 333MPa, no bending stress.
• yield strength = 415MPa, tensile strength = 550MPa.
• fracture toughness CTOD = 0.25mm (this is an arbitrary value for the purpose of illustrating the procedure, for the purposes of this example it is assumed to be independent of thickness and PWHT). Normally this value would be determined by fracture toughness tests of procedural weld plates.
• as-welded residual stress equal to yield strength of parent plate
• PWHT residual stress equal to 15% yield strength of weld metal. (Assuming heat treatment conditions described in BS 5500: 1994. [8] ).
• minimum defect depth for long defect to be reliably detected by NDT of 3mm.
• tolerable flaw depth to be determined by a PD6493: 1991 Level 2 procedure using TWI computer program Crackwise 2.

Results and discussion

The tolerable flaw size for the case described above, in both the as-welded and postweld heat treated condition are given in Fig.3.

Inspection of Fig.3 shows that the limiting tolerable depth of a long flaw in a girth weld is significantly greater in the PWHT than in the as-welded condition. This was expected. However, the increase in the limiting tolerable flaw depth was obtained by a reduction in residual stress, rather than by any change in the fracture toughness. If the flaw sizes for the as-welded joint are acceptable where the thickness is less than 50mm, there is no reason why the as-welded joint should not be acceptable for a plate thickness which exceeds 50mm, always assuming that there is no significant decrease in toughness.

As discussed previously, it cannot be assumed that the fracture toughness of the weld metal will always deteriorate with increasing section thickness. For this particular example, where the NDT threshold was set at 3mm deep, there would therefore be a case for avoiding PWHT, provided that studies of other possible flaws showed the surface breaking flaw to be the worst case. If the NDT threshold was increased to 5mm there would then be a case for PWHT regardless of section thickness. The above analysis illustrates limitations of applying PWHT on the basis of a thickness criterion alone.

In some cases the limiting tolerable flaw size can marginally decrease where the plate thickness increases. This is associated with a tendency for plastic collapse to become less dominant at the expense of an increased propensity to brittle failure (Fig.4). The increased propensity to brittle failure (indicated by an increase in the ratio and a decrease in the S _r ratio) means that the tolerable flaw size can, in certain circumstances, actually decrease with increasing plate thickness, without any change to the fracture toughness properties. PWHT plate is relatively less prone to brittle failure and governed more by plastic collapse than the as-welded plate. For different input parameters to those in the example, it is possible that the as-welded limiting tolerable depth could decrease rapidly with increasing wall thickness. This shows the importance of carefully defining the input parameters when calculating tolerable flaw size.

Summary

Traditional thickness criteria for PWHT has been shown to be unsatisfactory for a number of reasons. An alternative method for assessing the need for PWHT would be to use a fracture mechanics procedure as a selection criterion. The principal advantages of this technique are that PWHT can either be avoided, or alternatively, safety of the structure can be more reliably guaranteed. In some applications, such as on-site application of PWHT or repairs to existing structures, the savings in costs associated with the avoidance of PWHT make the use of fracture mechanics financially very attractive.

A flow chart and a simple example illustrate use of the fracture mechanics procedure. The example shows that, while PWHT will lead to an increase in the limiting tolerable flaw size for a given structure compared to the as-welded condition, the as-welded structure will often be acceptable even if the plate thickness exceeds the traditional thickness criterion. It is also recognised that assessment should be on a case by case analysis, rather than by the formulation of generalised rules.

This work was completed as part of TWI's core research project programme. It is suggested that the analyses used to justify exemptions from PWHT for offshore structures could be applied to other structures, such as pressure vessels, with considerable financial advantage.

References