API 510 Chapter 11- ASME VIII and API 510 Heat Treatment
Post-weld heat treatment (PWHT) is included in the API 510 examination syllabus, mainly in relation to vessel repairs. This makes sense as one of the roles of an API 510 inspector is to oversee repairs on behalf of the plant owner/user. Figure 11.1 shows the importance of this. Not all of the technical information relevant to PWHT is actually included in the API 510 document itself – most is contained in two welldefined (but separated) sections of ASME VIII.
This is not the end of the story. API 510 now adds (as it does with a few other topics) some requirements that can override the PWHT requirements of ASME VIII. The logic behind this is that whereas ASME VIII is a workshop-based construction code, under which PWHT can be done in a furnace under workshop conditions, API 510 deals with repairs, many of which will be carried out on site, where such closely controlled conditions are not possible. API 510 therefore provides easier alternatives that can be legitimately used during repairs. We will look at these parts of the content in turn.
11.1 ASME requirements for PWHT Just about all of the main examination questions on PWHT that relate to ASME VIII are taken from either sections UCS-56 or UW-40. They are well separated in the code document but are cross-referenced directly to each other.
11.2 What is in UCS-56?
UCS-56 contains a couple of pages of text surrounding a group of seven or eight tables. Figure 11.2 shows a sample. The main content is in the tables; their purpose is to specify PWHT temperatures and holding times for different thicknesses of material. Each table covers different P-groups of material – because simplistically, the P-group is related to the tendency of a material to suffer post-weld cracking problems.
11.2.1 UCS-56 table notes
Don’t ignore the half-page or so of notes underneath the various tables contained in UCS-56. They include information on either mandatory requirements or overriding exemptions, based mainly on material thickness.
Most exam questions (open book) will simply involve looking up the relevant PWHT time and temperature for a given material thickness in the correct ‘P-group’ table. Strictly, the material thickness to use is that of nominal thickness. This is defined not in UCS-56 but in UW-40 (f) – Fig. 11.2 shows the main points.
11.2.2 The UCS-56 text sections
There are a number of good open-book examination question subjects hidden away in the two pages of UCS-56 text. These relate to:
- The rate of heating of the PWHT furnace
- Allowable temperature variations in the furnace
- Furnace atmosphere
In addition to these clauses UCS-56 (f) and beyond gives six specific requirements relating to PWHT of weld repairs. In brief they are:
- The need for notification of repairs .
- Maximum allowable depths of repair weld (38 mm for P1 Grades 1, 2, 3 and 16 mm for P3 Grades 1, 2, 3 materials)
- Excavation and PT/MT examination prior to repair
- Additional WPS requirements
- Pressure test after repair
Be careful not to misunderstand these requirements – ASME VIII is a construction code only, so the repairs it is referring to in UCS-56 (f) are repairs carried out as part of the original manufacturing process, not repairs carried out after in-service corrosion or some other damage mechanism. For in-service repairs ASME VIII requirements are overridden by the less stringent requirements of API 510 section 8, which does not place any limit on repair weld depth and divides repairs into temporary and permanent types.
11.2.3 The UW-40 text section
Whereas UCS-56 covers the times and temperature requirements for PWHT, UW-40 describes the procedures for how to do it. There are eight main options, some more practical than others.
11.3 API 510 PWHT overrides
API considers it a major advantage to be able to override the ASME VIII requirements for PWHT. Remember the logic behind this – API 510 relates to vessels once they are in use where the practicalities of site working probably will not allow manufacturing shop conditions to be reproduced so easily, if at all.
API 510 section 188.8.131.52 says that, in principle, repair welding must follow the requirements of ASME VIII (it means UW-40 and UCS-56) but opens the door to two overriding PWHT alternatives set out in API 510 section 184.108.40.206.2. This subsection has been progressively expanded and elaborated over recent code editions – you can see this in the out-of-balance subdivisions in the code clauses (it goes to a concentration-popping seven levels of subhierarchy, e.g. section 220.127.116.11.2.2.1).
The two methods of PWHT replacement (section 18.104.22.168.2) are:
- Replacement of PWHT by preheat
- Replacement of PWHT by controlled deposition (CD) welding methods
These are shown in Figs 11.3 to 11.5. Both work on the principle that the stress-relieving effects of PWHT can be achieved (albeit imperfectly) by providing the heat required in some other way than placing the repair in a furnace.
11.3.1 Replacement of PWHT by preheat
As the name suggests, this simply involves replacing PWHT with preheating the weld joint and then maintaining the temperature during the welding process. The maintained temperature serves to give sufficient grain refinement to reduce the chances of cracking when the weld is finished and allowed to cool down. While this technique provides sufficient grain refinement it is clearly not as good as full PWHT, so it is limited to materials of P1(Grade 1, 2, 3) and P3(Grade 1, 2) designations. These have a low risk of cracking anyway, owing to their low carbon content. P2 Grade 2 steels containing manganese and molybdenum are excluded, as they have a higher potential for cracking.
API exam questions normally centre around the parameters and restrictions of the preheat techniques. These are listed in API 510 section 22.214.171.124.2.2.1, and illustrated in Fig. 11.4.
11.3.2 Controlled deposition (CD) welding
This is sometimes known as temper-bead welding and is described in some detail in API 510 section 126.96.36.199.2.3. The principle is simple enough – when one layer of weld metal is laid down on top of another the heat from the upper one provides some heat treatment (grain refinement) to the weld underneath. A multilayer weld which is built up in this way will therefore be given an amount of grain refinement throughout its depth. The top layer of the final weld pass will not have anything above it to provide it with heat treatment, so the solution is to grind it off. Figure 11.5 shows the idea.
API 510 (section 6 ) overrides ASME VIII PWHT Requirements API 510 Continues the principle of ifentifying alternatives to full PWHT on weld repaira it identifies 2 methods of replacing PWHT, depending on whether impact (Charpy) testing is involved
treatment, so the solution is to grind it off. Figure 11.5 shows the idea.
The CD technique is considered to be a little better at replacing full PWHT than the preheat only alternative. It is therefore used for materials where the specification requires impact (notch toughness or Charpy) testing as a condition of their use in pressure equipment. The fact that impact tests were required indicates that the material has a tendency towards brittleness so the preheat method would not be good enough.
These two PWHT replacement techniques, preheat and CD welding, have become a mainstay of API codes. They are now mentioned in API 510, 570 and 653 and, we can assume, are commonly used in practice, although more commonly in the USA than elsewhere. PWHT exam questions PWHT replacement questions seem to be well-represented in the API exam question book. Questions on the validity of the two techniques, times, temperatures and heat-soak band dimensions crop up time and time again.
Now try these familiarization questions.
11.4 ASME VIII sections UCS-56 and UW-40: PWHT familiarization questions