Stainless steel – how strong?

The issue of low elastic strength in stainless steels and non-ferrous alloys has raised its head several times, none more so than with B8 austenitic stainless steel. Data from the British Stainless Steel Advisory Service indicated that for structural purposes designers need to estimate a bolt capacity based on 59% of the specified 0.2% proof stress (PS) value.

This applies to fasteners in both solution treated (class 1) and work hardened (class 2) conditions. The following table compares specified 0.2%PS and corresponding elastic limit/bolt capacity values. It also shows the effect of increasing bolt diameter on bolt strength. O.2%PS and elastic limit reduce as the effect of work hardening of surface layers from the production process subsides with increasing bolt cross section. The drop off in strength is quite dramatic.

This has been confirmed by incremental, Load versus Extension tensile testing on full size bolts. Similar incremental tensile testing on other bolting alloys also shows significant differences in 0.2%PS versus true elastic limit. These alloys are in the heat treated condition.

Alloy 0.2%PS N/mm2 Elastic Limit N/mm2 Ratio %
B8 Cl 1 205 123 59
B8 Cl 2 ≤ 19mm 690 414 59
B8 Cl 2 ≥ 20-32mm 550 325 59
Monel K 500 655 475 73
Duplex 255 649 442 68
Marinel 21 A 720 487 68
Titanium Beta C 834 679 81
Titanium 6ALV4 ELI 830 593 71
Inco718 938 753 80

This actual lower bolt load strength potential raises a number of issues. Several non – ferrous alloys with potential usage in offshore operations are marketed on the basis of having similar strength to medium carbon low alloy steel B7, the staple energy bolt. Of course the strength similarity is based on 0.2% proof stress. Design/installation tightening decisions are made on this basis. When you see the ‘true’ strength potential the selected bolt in the replacement alloy quite often is not capable of delivering the design/installation loads needed. This is especially so where design loads and subsequent bolt diameter selection are made on the basis of utilising a high percentage of a bolt’s assumed yield strength i.e efficient design.

Issues are also raised if hydraulic tensioner tightening has been selected. The theoretical hydraulic overload needed to compensate for load transfer losses during the tightening process, resulting in a satisfactory, achieved, residual bolt load, might be feasible based on the specified 0.2% proof stress value but not be feasible when compared with true yield/elastic limit.

Another significant issue is on pressure containing bolted flanges where metallic/semi-metallic gaskets are used. These gaskets often require gasket seating stresses generated by bolt stresses of 200-300 N/mm2. This immediately means solution treated, austenitic stainless steels to Class 1, are not capable of delivering these design bolt stresses. Larger diameter, work hardened Class 2 stainless bolts may also not be feasible in such applications.

Lower than expected elastic bolt strength is not exclusive to stainless steels or non – ferrous alloys. Surprisingly it can be experienced in medium carbon low alloy steels too.