By Spencer Erling,
Education Director, SAISC
What a month May was for visiting American experts to visit South Africa to share their knowledge with us. Firstly there was Don Allen who came to talk about multistory light steel framed construction (see article on page 18) and then we were lucky enough to have Robert (Bob) Shaw, an expert on the AWS welding specifications and bolting issues, who did a number of one day workshops in Cape Town, Durban and Johannesburg on the welding issues and one talk in Johannesburg on the bolting issues.The seminars were hosted by the Southern African Institute of Welding (SAIW).
This article is a summary of the welding workshop. Bob emphasised the differences between the ASME (American Society of Mechanical Engineers) boiler pressure vessels and piping standards and the AWS structural standards. This article concentrates on structural issues.
TOPIC 1: STANDARDS AND ACHIEVING THE RIGHT QUALITY
To start with Bob tied together AWS, SANS10162 design code (which is identical to the Canadian CSA16), SANS2001 CS1 construction requirements and then went straight to the quality requirements confirming that the (structural steel) contractor is required to do all inspection requirements of the above documents and the engineers specifications including (but not limited to):
1. Inspection prior to welding
a) Welder qualifications
b) Welding equipment
c) Consumables
i) Welding rods and their storage
ii) Gasses
d) Suitable weld procedure specifications (WPS) for the proposed joints
e) WPS settings for the machines (current, voltage, wire feed speed, gas flow etc)
f) Surface condition of steel to be welded
i) Free of discontinuities
ii) Unwanted dirt i.e. rust, grease, oil, paint etc, thermal cut edges cleaned
iii) Condition of tacks, removed where specified
g) Fit-up of joints, root openings, weld access holes, condition of backing strips. Details of allowable inaccuracies of joint alignment were covered later in Bob’s talk. The main ones are:
i) Up to 2mm gaps in fillet welds (without increase in weld size) up to 5mm by increasing the weld size by the size of the gap. From 5mm to 8mm it is required that a backing strip be used.
ii) For groove welds, root openings can be ±2mm out of tolerance, the angler of the groove -5° to +10°. iii) Wind velocity, ambient temperature, minimum and maximum preheating requirements
2. Inspection during welding
a) WPS setting in production
b) AC/ DC positive
c) Travel direction for vertical up
d) Interpass temperatures
e) Changes in ambient conditions (wind, rain etc)
f) Welding techniques (electrode angles, stringer beards, sequence etc)
g) Interpass cleaning, inspection of profiles, NDT if required
3. Inspection after welding
a) Fabricated member tolerances (web/ flange distortion etc)
b) Delay before welding can be inspected (cooled properly to ambient temperature)
The requirements for visual inspection in AWS
Table 6.1 of AWS describes the requirements fully for visual and dimensional inspection. Bob discussed these in detail under
Topic 9. Other than to generically describe them below, no details will be covered in this article.
Table 6.1 is (like the rest of AWS D1.1) very clear in what the visual inspection requirements are and is covered in the Red Book with some detail.
a) Weld free of cracks
b) Weld size length and location
c) Weld profile
d) Weld appearance (appears to be fused where visible, overlap), weld craters, undercut, and porosity
e) Shape of beam notches (copes) – radius corners with no nicks in web
f) No unspecified welds, arc strikes, tack welds or construction aids (all ground smooth and repaired after removal)
g) Re-entrant corners
h) NDT if called up i) Repairs, where required, to repair procedures
j) Welders ID/ weld map
k) All documentation in the data file, release notes
Quality plan (quality assurance system in American terms)
Quality plans have been covered in detail before in Steel Construction (copies can be obtained from spencer@saisc.co.za) so will not be repeated in this article.
Nevertheless, please “responsible engineer”, be reminded that at the end of your project, in terms of the National Building Regulations you will be required to sign off the project as having been built to your design. I do not know of any method, other than a good quality plan (that is signed off as each activity has been properly completed), by which an engineer can possibly
ensure that every detail as required in the design, the drawings, the engineers specifications, national codes and specifications and NDT testing are attended to correctly. It is at your risk to run any engineering without quality plans!
Bob spent some time on the document AISC360 which is their specification for structural steel buildings. Table N5.4-1 covers the inspection of welding (most of which is summarised above).
He then proceeded to explain that if your company is doing all the steps requiring inspection as described above properly, then the step to getting accreditation to ISO3834 quality requirements for fusion welding of metallic materials (welding process quality management) is not a big one.
I am sure that most of our bigger fabricators are aware that, in order to do work for Sasol, ArcelorMittal and Eskom (amongst others) in the future, you will be required to be ISO 3834 accredited (by the South African Institute of Welding – contact Sean Blake sblake@saiw.co.za). Consulting engineers in South Africa should also start specifying requirement for this accreditation to be sure that the welding is done properly on their contracts.
There are AWS specifications that set standards to cover the requirements for welding inspector qualifications. The Canadian welding bureau also has specifications for certification of welding inspectors. Once again the SAIW runs courses for such inspectors and keeps a register of such qualified persons.
TOPIC 2: ‘WELDING NOMENCLATURE’
All the details will not be repeated in this article but a few are worth noting
1. CJP groove welds
a) ’Complete joint penetration’ is the AWS term for what is loosely called a ‘complete penetration butt weld’ in South Africa.
2. Fillet weld terms, groove weld terms and welding positions
Whilst these were discussed in detail, figures 1 – 4 (page 44) say it all…
3. Bob described the specification EN1011-2:1998
a) Avoidance of hydrogen cracking was explained using figure 5 and clauses used to calculate preheat temperatures, units of cracking susceptibility (UCS) and how to use the UCS results i.e.
b) UCS >30 low resistance to cracking; UCS <10 high resistance to cracking
c) Risks of cracking
TOPIC 3: MATERIAL STANDARDS (very briefly)
1) The welding process were described (not repeated here)
2) Welding consumables (not repeated here, but adequately covered in the Red Book for non-alloy materials) and their care
3) Welding gasses
TOPIC 4: WELD PROCEDURE SPECIFICATIONS; TOPIC 5: WELD PROCEDURE QUALIFICATION AND TOPIC 6: WELDER
QUALIFICATION
As the SAISC recommends that weld procedure specifications should be drawn up by specialists (welding engineers in particular) the details will not be covered in this article. The same specialist should assist in the qualification of the procedure and the welders. For typical workshop welding requirements a ‘3G’ qualification is very suitable for welders since the test which is done in a vertical up direction, covers the welder for fillet and groove welds in plates up to 20mm thick in the flat, horizontal and vertical up welds. This qualification lasts indefinitely unless the welder has not done the procedure for more than six months or if there is some reason to question the welder’s ability.
TOPIC 7: WELDED JOINT DETAILS
AWS has a multitude of so-called prequalified joints within the text that can be used for weld procedure specifications without further testing for a whole range of American steel grades. There is a simplified table of some of these details in the Red Book. Pedantic inspectors call for procedure re-qualification on the basis that South African steel grades are not covered by AWS. Future South African construction specifications (SANS2001 CS1) will have a clause that allows pre-qualified AWS weld details to be used with South African grades of steel.
TOPIC 8: FABRICATION DETAILS AND QUALITY
Weld access holes were discussed as per figures 6 and 7 (page 46).
The use of backing strips for welding CJP groove welds from one side was discussed. AWS details show how steel backing strips can be used and removed if required. AWS prequalified welds do not apply to copper and ceramic backing strips and procedures need to be re-qualified.
Extension pieces (run-on and run-off plates [figures 8 and 9] in SA welding tabs in the US) are recommended for continuous fillet welds to eliminate craters at the start and end of important welds. The requirements for removal methods (i.e. grinding and not breaking them off) were also described.
Tack welds play an important role in steel fabrication. Boilermakers should be qualified to do tack welds. If the tack weld is to be
incorporated in the finish weld, then it must be done to the same procedure as the finish weld, failing which it should be removed before welding.
It is also pre-scribed that no welding be done at temperatures below -20°C (not an issue in South Africa), nor on wet or ice covered surfaces, nor in high wind conditions.
TOPIC 9: VISUAL WELD ACCEPTANCE CRITERIA
As mentioned above Table 6.1 of AWS, describes AWS requirements fully and accurately and are not repeated in this article.
What is of interest to note is that the EPRI (Electric Power Research Institute) has published a document called “Visual Weld Acceptance Criteria” which is available as a free download at hhtp://my.epri.co,/portal/server.pt?Abstractid=NP- 5380-V1-V3(SET). The requirements are less onerous than AWS and has gained acceptance in the USA.
Workshop welding inspectors should be properly trained in the requirements of Table 6.1. Design or the responsible engineers need to understand this document as well.
TOPIC 10: NON DESTRUCTIVE TESTING REQUIREMENTS AND CRITERIA
Some information is covered in the Red Book. Details will not be covered here. Once again, consult an expert if exceptional quality of welding is required for advice on what testing methods apply and their relevant acceptance criteria.
The AISC N5 document mentioned above categorises buildings by risk associated. They are described by risk category from IV (essential buildings or high risk associated with collapse) down to risk category I (low risk to human lives). The document then advises on NDT by category.
TOPIC 11: SUITABLE INTERVALS FOR WELDING INSPECTION TASKS
The most important item covered in this section was a pie chart summary of case studies of weld inspection and testing which highlights the following major contributing factors to failure in quality of welds:
1) 18% Poor or incorrect fit up
2) 16% welding conditions (position accessibility)
3) 27% welders’ skills
This summary is a simple guide where to spend your time checking.
IN CONCLUSION
I am sure that every person who attended the Bob Shaw workshops, irrespective of their interest in welding, i.e. designer, fabricator, inspector, end user came away having learnt a tremendous amount. Speaking for myself, it was a pleasure to listen to Bob’s easy style of delivery and learn so much at the same time.
As South Africans we are truly grateful that Robert Shaw came to South Africa and shared with us his extensive knowledge of welding
