Australian Pressure Equipment Knowledge

Basis of PE Material Grouping

Details

Category: Design, Materials, Manufacture and Installation

Prepared for:  AICIP, ACA, AIES, AINDT, APIA, CAAA Eng Aust, MA, NATA, OGF, SA, WTIA, Industry and Regulators

1 Scope
This note provides guidance on the basis of the proposed material grouping in the current revision of AS/NZS 3992 - Table 5.3.Parent Material Grouping.

2 Background
.1 AS/NZS 3992 is being reviewed to upgrade requirements to cater for the changes and advances in welding technology and global requirements for pressure equipment (PE). The revision aims to improve the overall efficiencies, performance and economy of the PE Industry (makers, users and repairers).
.2 This note is intended to assist the Standards Australia Committee ME-001-15 and related committees; and to provide a draft for a modified note which explains the basis of the finally issued Table 5.3.
a) clarify the basis of the material grouping system used in AS/NZS 1200 Standards;
b) review recent problems of the grouping of high strength pipeline type steels and quenched and tempered C and C-Mn steels ie Groups A3 and A4 (and G).

3 Scope
.1 This table is aimed to be the key reference for grouping of materials for AU PE Standards.
.2 At this stage covers only metallic materials:
a) listed in AS 1210, AS 1228, AS4041 and AS/NZS 3050.
b) selected overseas material standards used in AU made or used PE.
c) The grades listed in the previous edition of AS 1548 (as these may still be available for use in new or repaired PE).
.3 Grouping for brazing is introduced in this edition.

4 Purpose of Grouping
.1 To facilitate to use “equivalent” alternative materials ie which when using the same WPI will have almost the same or better weldability and provides joints with almost the same or better mechanical and chemical properties.
.2 The welding procedure provides for various conditions such as pre and post forming, heat treatment and other treatments. This grouping is also used for specifying requirements for limits of forming and heat treatments and for different NOT for normal metallic and for fine crack susceptible materials.
.3 To provide a common flexibility in sourcing of suitable material but avoiding delays and effort in re-qualifying (ie pressure) the material for the same welding procedure.
a) Simplify specification and reference to a variety of detailed technical requirements applicable to a range of similar materials; and
b) Reduce cost and effort of qualifying welding and brazing procedures and welders through use of the same procedure for a range of materials in the same group.
.4 For use with any suitable welding process or procedure by suitable welders and welding equipment.
.5 For use where preferable for any PE or other weldment to any product Standard or Code (possibly in some specific case with minor variation).
.6 For brazing similar purposes to share for welding.
.7 Overall to improving and performance of PE and other industries and to conserve energy and resources and minimise pollution.

5 Basis of Grouping
.1 Originally, to align with the Draft ISO Pressure Vessel Standard in 1970, which was somewhat similar to ASME IX and German AD groups.
.2 Now, to align as far as practicable with world best practice ie ISO, EN and ASME groups. IT is understood that CEN and ASME are currently working on this.
.3 Firstly, the Groups are based on COMPOSITION – to provide approximate compatibility of fabrication and use eg similar welding procedures and similar high and low temperature and corrosion performance.
.4 Secondly, on STRENGTH – to cater for different critical properties within the same composition group to give approximate equivalent strength and hence equipment thickness.
.5 Thirdly, where necessary on WELDABILITY – to cater for different material features (eg carbon equivalent and heat treatment condition) which significantly influence welding variables and thus the controls needed to ensure appropriate:
a) freedom from hydrogen and other cracking (eg by preheat or low hydrogen welding to avoid HAZ cracks, or by more sensitive NDT for crack-sensitive steels);
b) HAZ ductility (to provide for __________ and for energy absorption in an overload or accident);
c) HAZ notch toughness (to avoid brittle fracture);
d) HAZ hardness (to provide corrosion or wear resistance and quality control).
e) welding or brazing procedures proven for one material can be safely used with certain other materials in the same group without re-qualification, but possibly with minor variations and ensuring weld soundness is achieved.
.6 Excluding various cast irons as these are rarely welded in pressure equipment.
.7 For non-ferrous metals – this is the same as adopted in ASME Section IX, which in turn is being considered by CEN. IT is consistent with c) above but is based only on COMPOSITION.
.8 Appendix C gave the basis used for steels which completely aligned with the grouping in the proposed ISO Premium _________ Standard in 1970. It has been only slightly modified to very closely equate to ASMEIX and partly with ISO/TR 15608 (assured in EU) to facilitate.
.9 It provides for use of a proven (ie qualified) for other materials (type, grade and condition) which have:
a) Almost the same or better weldability or brazability;
b) Comparable or compatible composition; and
c) Almost the same or better mechanical properties in the completed point.
Such grouping should have negligible effect on “soundness” of the welded joint which resulted in material groups adopted in:
a) Past and current global Australian Standards of welding.
b) ASME BPVC-IX
c) ISO/TC 1560S
d) The original ISO proposed for PE which is adopted in EN-13445 –Unfired Pressure Vessels.
e) AS/NZS 1554 and mining other standards.
.10 It is based on sound science/metallings and mechanical proven by __________ successful global tests and experiences.
Group A (Carbon and Carbon-Manganese Steels)
.1 This group of steels covers over 80% of welded metals. The strength and properties of their welded joints vary greatly with carbon equivalent and heat treatment condition. Thus it has been necessary to make sub-groups to cater for these variables.
.2 The current concerns are:
a) Group A3 composition limits – particularly carbon equivalent (CE) and whether this group should include quenched and tempered carbon and carbon-manganese steels, including those with micro-alloying elements which influence strength etc, eg boron; and
b) the introduction of a new Group A4 for Q & T carbon and carbon-manganese steels to cater for some ASME, Japanese and British steels; and
c) should the stringent requirements for Group G apply to Group A4.
.3 Table 1 shows current grouping together with ASME IX and ISO 9986-3:1995, and the suggested grouping to cover the above concerns.
a) These proposed changes aim to simplify the grouping where possible, have the scope to cover all such steels, minimise change to essentials, and possible align closely with ISO and ASME.
b) The main change proposed for Group A3 is to adopt CE = 0.45 in order to cover API 5L pipe Grades x52, x60, x65, x70, x75, all of which have Re over 350 MPa, are fine grained and micro-alloyed and have some thermo/mechanical treatment to give the fine grain which results in high yield strength. Also included in this group are similar steels within AS 1594 ie Grades XF400 and XF500. All in this Group are made by strip mill. [Note: x52 above indicates 52000 psi yield and XF400 indicates 400 MPa tensile strength].
This Group does not include quenched and tempered steels.
The main feature of these steels is their inability to be hot formed or normalised without some loss of properties unless later Q&T’ed.
Any variations for the small differences in carbon equivalents should be provided for in AS 4458 eg the preheat required for cutting or welding and the temperature for normalising is already given for different carbon or carbon equivalent values within Groups A1 and A2. Thus further sub-groups for minor differences in carbon or CE are not needed.
Virtually all steels within Group A3 have CE less than 0.4 and maximum thickness is usually less than 15mm for API 5L and 7mm for AS 1594 thus facilitating flexible welding procedures except where strength is a major requirement.
This minor change in CE should have no impact on industry other than helping to use available materials more efficiently.
c) The other main change to Group A is the proposed new steel Group A4. This group is identical with ASME IX P No 1.4. It is restricted to those C and C-Mn (including C-Mn-Si) steels which are quenched and tempered to enhance properties. They are not regarded as low alloy steels but may only have micro-alloying elements (which includes boron as in ASTM A 724 Grade C, the JIS G3115 SPV 490 and the UK RQT 601 or 701 – the latter having Rm = 790).
Features of this group are their higher strength and ability to lose HAZ properties with excessive heat inputs and temperatures. They also cannot be hot-formed or normalised without further quenching and tempering.
A difficult problem in Group A4 is the limit for Rm and it is suggested that 790 MPA be adopted as this is the highest value known or likely for use with pressure equipment. However the conditions of use should make it clear that where Rm is over 620 MPa, Group G conditions apply. This would align with ASME for Rm over 620.
Quenched and Tempered Steels Groups A4 and G
.1 Early Q&T low alloy steels Group G which had bad experiences primarily with hydrogen and stress corrosion cracking. This led Europe to ban such steels for pressurised transport vessels.
However today steels and know-how have improved to the extent that reports of problems are rare. Some relaxation is now justified for those improved steels.
.2 Group A4 steels differ substantially from Group G steels and have been used for over 5 years without any reported incidents. It is believed that leaner chemistry and lower strength have justified the more relaxed practices used for small to large vessels including road tankers. This is the basis of the relaxations proposed below for A4 and G in a future amendment to the relevant Standards ie AS 1210, 4041, 3992 and the imminent Manufacturing standards.
.3 Recently ASTM A106 B tubing has been used in the quenched and tempered condition to reduce erosion. This could be classed as A4 but needs special requirements to ensure safe performance.
.4 The various manufacturing requirements for A4 and G steels need to be revised, simplified and unified in the revision or new Standards.
.5 This is complex and proposals differ from the appeal of simplicity in ASME treating A4 and G as equals for use requirements. The modifications are considered reasonable for A4 with Rm not over 620 MPa and for G steels as shown and take into account latest Japanese practice.
Weld procedures qualified with one of the steels in a Group, covers the lower alloyed or lower strength steels in the same group provided the welding consumables used can be used with both steels. Group A2 covers Group 1 steels.

6 Limitations
.1 The grouping only partly indicates comparable performance of _________ welded joints (using the same WPS) against various types of corrosion, erosion, low or high temperatures, high radiation or ballistic or aesthetic performance.
.2 Grouping (of all systems) does not mean that all materials in the group can be substituted for that use in the qualification test without ensuring compatibility to meet required properties and service.

7 Application
The table is designed to:
a) apply to any welding process for any standard local or overseas PE, pipe lines or structures, machines etc.
b) apply to any metallic material (and combination) by direct identification or by determining equivalence of composition, and properties and successful tests and experience.
c) provide for additional specific requirement for:
- special cases eg __________ or ballistic services where the grouping may not be sufficient and successful proving test or experience is warranted and specified.
- use of industrial material, high radiation, corrosion., hydrogen.
- complete exact compliance with another Standard or Code.

8 Recommendation
.1 Interested parties review the proposals above for grouping and use requirements, and reach a consensus keeping in mind the significant differences between A4 (Rm not over 620 MPa) and G steels.

9 Effect of Change
Again this addition itself should have little impact on industry other than cover relatively new steels for which no specific requirements are given.
The contributions from a number of organisations and persons has been invaluable in this investigation and is appreciated.

10 References
.1 ASME BPVC Section IX Welding & Brazing Qualifications.
.2 ISO/TR 15608:2005 Welding – Guidelines for a metallic materials grouping system.
.3 150/TR 20173:2008 Grouping Systems for Materials – American materials
.4 AS 1210
.5 ISO 9956-3 1995
.6 AS 4037
.7 AS 3992

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