ase Studies

High Strength Stainless Steel Fasteners
For Pressure Vessel Applications

Engineering is a progressive undertaking and usually a follow-on event from previous innovations. With the more recent development of fasteners manufactured in high strength stainless steel it’s a fact that outstanding technical performance has produced significance prospects for applications in adverse environments, and not least, the advancement in critical bolted joint technology.

These stainless steel alloys, from the austenitic category, have been developed to meet the demand for high strength durable fasteners, with full traceability. The versatile BUMAX® range of products are well established and have proven to be a balanced range of strength grades that complement the more popular standard stainless steel fasteners normally used in general applications.

Environmental Requirements are Here!

In an ever-changing world, environmental issues play an important part in ever day life. Protecting the planet is a fundamental necessity, which is being backed up with resolute legislation. No doubt this will be an ongoing situation with evermore directives being put in placed.

Against this background, other issues develop. The need to enact health and safety regulations that protect the principles of conforming to good engineering practise is just one aspect. Recent major incidents in the off-shore industry, where miss understandings have arisen, leading to a case when incorrect installation torque was applied to a bolted flange assembly. Due to poor training and a lack of knowledge about fastener technology, unfortunately resulted in a failure. This is just one incident that has required a European Union Directive in order that safe operating conditions would prevail.

Why Apply the Pressure Equipment Directive Now?

The European Pressure Equipment Directive (PED) No. 97/23/EC came into force on 29 November 1999. For a period of 30 months until 29 May 2002 implementation and application is voluntary however from 30 May 2002 all items of pressure equipment being placed on the market in the European Union must comply with the Directive and evidence of compliance by carrying the CE marking as applicable.

Legislation across the European Union requires that all applicable items of pressure equipment being placed on the market by May 2002 must be fully compliant with the PED. As a result manufacturers must revise their working practice and develop the appropriate Technical Files for their full product range prior to this date. As a result preparation cannot commence soon enough since the PED encompasses design verification, material selection, manufacturing/fabrication practices and qualification, product testing, product marking and user instruction compilation.

The development of BUMAX 88 stainless steel material by Bulten Stainless AB now enters a new market era as a result of being an approved material for critical bolted joints.

The EU Directive on Pressure Equipment is not clear and easy to understand on bolted joints, but for the specifier it presents an opportunity to select a corrosive resistant fastener, which will provide high strength integrity fastening solutions. BUMAX 88 complements the existing generic range with full traceability, however as a result of this development Bulten Stainless will be changing product marking in order to conform to the generic family nomenclature.

Development - The Next Level of Customer Requirement.

Responding to a market change, BUMAX 88, a new stainless steel fastener strength grade from Bulten Stainless has been developed for pressure vessel applications and has passed all the necessary test requirements for TÜV approval. This development is in accordance with the new European Union Directive; Pressure Equipment Directive PED 97/23/EC and the previous standard for bolted joints for pressure vessels AD-Merkblatt, AD-WO/TRD100.

For the design engineer it will provide an significant advancement to a yield strength level of 640 MPa, instead of the 450 MPa that was specified previously and the 200Mpa specified in EN 10269 Table 4 which is refereed to in PED. In this case, an increase in strength of 42% compared with the previous AD - Merkblatt and 320% compared with EN 10269.

Memory Reminder.

Megapascal (MPa) is the valuation of stress or pressure. A Pascal (Pa) equals one Newton per square millimetre (N/mm).

For mechanical fasteners, stress is expressed in megapascals.

High Strength Stainless Steel Fasteners

Product Range and Technical Information.

The product types manufactured in BUMAX 88, and approved for pressure vessels in a property class higher than A4-80, and for use in the temperature range of minus 200ºC up to 400ºC, are as follows..

  • BUMAX 88 to ISO 4014 and 4017 screws and bolts M6 to M24 with lengths greater than 3 diameters.
  • BUMAX 88 to ISO 4762 hexagon socket cap screws M6 to M24 with lengths greater than 3 diameters.
  • BUMAX 88 Studs M6 to M24 with lengths greater than 3 diameters.
  • BUMAX 88 ISO 4032 hexagon nut M6 to M24.

The mechanical properties are as follows.

  • Tensile strength Rm. min 800 MPa.
  • Yield strength Rp0, 2 min 640 MPa.
  • Proof load for nuts is the tensile strength for corresponding bolt or screw equal to 800MPa.
  • Elongation minimum 0,4xd.
Tightening Torque Nm.
M6 M8 M10 M12 M16 M20 M24
10 25 47 82 198 385 665

Yield Strength at Elevated Temperature.
100° 200° 300° 400°
>510 MPa >480 MPa >450 MPa >420 MPa

Other members of the BUMAX family not especially approved for pressure vessel applications are available as follows.

  • BUMAX 88 Spiralock all metal lock nuts.
  • BUMAX 88 hexagon head bolts and screws up to M36.
  • BUMAX 109 hexagon head bolts and screws and hexagon socket cap screws M6 to M16 (strength similar to 10.9 carbon steel bolts).
  • BUMAX 109 to ISO 4032 hexagon nuts M6 to M24 (strength similar to 10.9 carbon steel nuts).
  • BUMAX Hard Taptite® with surface hardness ca 1200 HV.
  • Washers in hardness 200 and 300 HV for BUMAX 88 and 109.

Submitted by FERA Member Arne Andersen - Technical Manager Bulten Stainless Industry AB.

For further information contact e-mail:

BUMAX® is the registered trademark of Bulten Stainless Industry AB.

Taptite® is the registered trademark of REMINC.

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