The question is sometimes asked ‘Can I use our existing welding procedure qualifications?’ where the qualification specification required by the contract is one that has not previously been used by the organisation. This is particularly relevant when substantial costs and/or delays will be incurred if re-qualification of the welding procedures is necessary. The two most frequently encountered specifications are ISO 15614 Part 1 and ASME IX and whilst these are written with the same purpose (that of giving assurance that a welding procedure will provide the desired joint properties) there are major differences between the two specifications that mean that they are not equivalent. It will not be possible in this short article to cover every welding variable and its range of approval in the two specifications. Where compliance is required then reference MUST be made to the appropriate specification.
With respect to ASME IX the specification requirements can be applied in two ways; ASME intent and ASME stamp. If the welded item is to be ASME stamped this can only be done by a manufacturer who has a quality system accredited by ASME and who holds an appropriate stamp, N stamp for nuclear components, U for unfired pressure vessels, S for power boilers etc. All the requirements of the ASME specifications MUST be complied with, even to the extent of dimensions of the mechanical test pieces and the calibration of testing equipment.
ASME intent is used where the item is not to be code stamped but is perhaps only designed to the relevant ASME code and some flexibility is possible with respect to the manufacturing aspects of specification compliance. Such flexibility may allow the manufacturer to submit to the client or inspecting authority procedure qualification records (PQR) to ISO 15614 Part 1 for approval that can be shown to be technically equivalent to an ASME PQR.
ASME IX covers the qualification of welders and welding operators, welding procedures, brazing operatives and brazing procedures for the complete range of ferrous and non-ferrous engineering metals (steels, copper, nickel, aluminium, titanium and zirconium alloys) and oxy-gas, arc, power beam, resistance and solid phase welding processes. ISO 15614 Pt1 covers the welding procedure qualification of arc and gas welds in steel and nickel alloys only. Other alloys and joining processes are covered by additional specifications within the ISO 15614 series.
Both specifications identify essential variable (although ISO 15614 Pt1 does not describe them as such) to each of which is assigned a range of approval. A change to an essential variable outside of its range of approval requires the welding procedure to be re-qualified. ASME IX in addition identifies supplementary and non-essential variables. Supplementary variables are only invoked when toughness requirements are specified by the application code, eg ASME VIII or ASME B31.3. Non-essential variables, as the name suggests, are those variables that are not regarded as affecting the quality or mechanical properties of the welded joint and comprise such variables as the weld preparation, shield gas flow rate, method of back gouging, shield gas nozzle size etc. Although these variables are non-essential it is a requirement that they should be referenced on the welding procedure. It is therefore NOT acceptable to use a butt welding procedure to specify how a fillet weld should be made.
ISO 15614 Pt1 does not identify any variables as non-essential; where a variable is not regarded as significant it is simply not referenced in the specification. There are several variables in both specifications where there is no range of approval; the manufacturer, the welding process and the application or deletion of post weld heat treatment (PWHT) for example.
In order to reduce the amount of qualification testing, both specifications group alloys of similar characteristics together. Qualifying the welding of one alloy within the group allows the other alloys within the group to be welded. ASME IX assigns the groups numbers with steels being numbered P1 to P15F. Any alloy that does not have a P number is regarded as unassigned; a procedure qualification carried out using an unassigned alloy qualifies only that specific designation of alloy. Until recently only alloys that complied with the ASME and/or ASTM material specifications and/or had a UNS number were assigned P numbers. However, a limited number of EN, Canadian, Chinese and Japanese alloys have now been introduced into the list of assigned alloys.
ISO 15614 Pt. 1 also groups steel and nickel alloys into families with similar properties but is somewhat less prescriptive than the ASME code in that, provided alloys have similar chemical compositions and mechanical properties, the material specification is not relevant – for example a plain carbon steel with less than 0.25%C and a minimum specified yield strength less than 460MPa falls into Group 1 irrespective of whether or not it is a pressure vessel or structural steel or supplied in accordance with EN or ASTM material specifications. To determine into which group the alloy falls reference should be made to ISO/TR 15608, the specification that lists both ferrous and non-ferrous alloys and assigns them a group number.
Other significant differences between the two specifications with respect to the arc welding processes are:-
- ASME IX requires only tensile and bend tests to qualify a butt weld. ISO 15614 Pt1 requires a far more extensive test programme of visual inspection, radiography or ultrasonic examination, surface crack detection, tensile and bend tests and macro-examination. In certain circumstances Charpy-V impact tests and hardness surveys are also required.
- ASME IX specifies that the tensile strength of the cross joint tensile specimen shall be at least that of the minimum specified for the parent metal and that bend test coupons should have no discontinuity greater than 3mm. ISO 15614 Pt1 has identical requirements for these mechanical tests but in addition specifies an acceptance standard for the non-destructive testing; impact test results, when required, that match the parent material toughness and hardness limits when hardness testing is required.
- ISO 15614 Pt 1 requires Charpy-V impact testing for steels over 12mm thick when the material specification requires it. ASME requires impact testing only when specified in the application standard. This requirement makes heat input a supplementary essential variable in ASME IX but an essential variable in ISO 15614 Pt1.
- Hardness testing is required by ISO 15614 Pt1 for all ferritic steels with a specified minimum yield strength greater than 275MPa. A maximum hardness for joints in either the as-welded of PWHT’d condition is specified. ASME IX does not require hardness testing.
- ASME IX allows a reduction in preheat of 55°C before requalification is required. ISO 15614 Pt1 does not permit any reduction in preheat from that used in the qualification test.
- ASME allows the maximum interpass temperature to be 55°C above that measured in the qualification test. ISO 15614 Pt 1 permits no such increase.
- ASME IX requires pressure containing fillet welds to be qualified by a butt weld procedure qualification test. Non-pressure retaining fillet welds may be qualified by a fillet weld test only. ISO 15614 Pt1 requires a fillet weld to be qualified by a butt weld when mechanical properties “.... are relevant to the application...” i.e when it is a load carrying fillet weld. In addition, whilst a butt weld will qualify a fillet weld “....fillet weld tests shall be required where this is the predominant form of production welding...” i.e. an ISO compliant welding procedure where the majority of the welding is of load carrying fillet welds must reference both a butt weld and a fillet weld procedure qualification.
- Weld metal transfer mode, where relevant, is an essential variable in both ISO 15614 Pt1 and ASME IX but the current type is an essential variable in ISO 15614 Pt1 and a supplementary essential variable in ASME IX.
- A change from manual to automatic welding is an essential variable in ISO 15614 Pt1 but a non-essential variable in ASME IX.
Whilst there are several other variables in the two specifications that have substantially different ranges of approval there are many that have ranges that are very similar – material thickness being but one example. This article has highlighted some of the significant differences but to ensure that the welding procedure and its supporting procedure qualification record are compliant the specifications must be referred to. The answer to the question posed at the start of this article is therefore – it depends upon what you can persuade the client and inspecting authority to accept!