Welding of ferritic cryogenic steels

Job Knowledge 100

The nickel content ranges from around 1.5 to 9%, although there are some fine grained carbon-manganese steels that may be operated at temperatures as low as -50°C. These grades of steel are generally found in the oil and gas and petrochemical industries where they are used for the handling and storage of liquefied petroleum gases (LPG) at temperatures down to approximately -100°C and, in the case of the 9% nickel steel, down to -196°C. They are also found in the gas processing industry for the production and handling of gases such as carbon dioxide and oxygen as shown in Table 1.

Table 1. Approximate minimum service temperatures and applications of the cryogenic steels

The choice of which steel to use for any particular application depends not only on the temperature but also on such aspects as section thickness required by design and the possibility of stress corrosion.

The applications of these steels require that the mechanical properties, in particular the toughness, of welds and their associated heat affected zones match or are very close to those of the parent metals. The fabrication of the cryogenic steels into pipework and vessels therefore requires careful selection of welding consumables and close control of welding parameters.

Manual metal arc (MMA) electrodes matching the composition and Charpy-V impact strength of the fine grained carbon manganese steels at -50°C can be obtained, for example, AWS A5.5 E7018-1 electrodes, although the addition of a small amount of nickel, up to 1%, will give added confidence in achieving the required toughness. Matching C/Mn composition metal active gas (MAG), flux cored (FCAW) and submerged arc (SA) consumables will not give adequate toughness at -50°C and require nickel to provide the required as-welded toughness.

This is generally limited to a maximum of 1%Ni to comply with the NACE International ISO15156-2/MR0175 requirement for use in sour service. For even greater confidence that acceptable Charpy-V values can be achieved and to provide an improved tolerance to procedural variations then 2.5% nickel containing consumables may be used.

The 1.5%Ni and 2.5%Ni steels may be welded with 2.5% Ni consumables and these will provide adequate toughness down to -60°C in both the as-welded and post weld heat treated (PWHT) condition. A word of caution, however; the tensile strength of PWHT'd TIG and MAG weld metal may fall below the minimum specified for the parent metal. MAG weld metal deposited using a shield gas with a high proportion (>20%) of CO₂ appears to be particularly sensitive.

Consumables are available for the MMA and SAW welding processes but not for the TIG, MAG or FCAW processes.

For depositing TIG root passes in the 3.5 Ni alloys, a 2.5% Ni filler metal is normally used. Although the 3.5% Ni consumables are capable of providing adequate toughness at -101°C they are very sensitive to variations in welding parameters, heat input and welding position. This sensitivity results in a wide variability of impact test results so for the more demanding applications, alternative nickel based filler metals such as AWS ENiCrFe-2 or EniCrFe-3 are often used enabling all of the conventional arc welding processes to be used.

The 5% Ni and 9% Ni alloys are conventionally welded using a nickel based filler metal. 6.5% Ni MMA electrodes are available but these are not capable of consistently providing adequate toughness much below -110°C. Consumables for welding the 9% Ni alloy have been developed; these typically contain 12% to 14% nickel. However, the cost of production is such that they do not compete with the nickel based alternatives.

A problem with the nickel based consumables that were initially used to weld these steels is that their tensile strength is substantially less than that of the parent metal. Higher strength fillers of the AWS EniCrMo-3 (alloy 625) type are now readily available and these enable all the arc welding processes to be used. They also match parent metals with respect to toughness and ultimate tensile strength although the 0.2% proof strength of TIG, MIG and SAW weld metals may fall below that specified for the 9%Ni steel.

As with any steel where good toughness is required, heat input must be controlled. It is recommended that interpass temperatures are limited to a maximum of 250°C and ideally less than 150°C for the 9%Ni alloy. Heat input from welding should be limited to approximately 3.5kJ/mm for SAW and 2.5kJ/mm for MMA.

Preheat may be required for the carbon-manganese and up to 3.5% Ni alloys, depending upon section thickness, joint type and restraint to reduce the risk of hydrogen cold cracking. ASME B31.3, for example, recommends minimum preheat temperatures of 79°C for carbon steels greater than 25mm thick, 93°C for all thicknesses of the 1.5%, 2.5% and 3.5% nickel steels but only 10°C for the 5% and 9% Ni alloys. The reason for this low preheat temperature is that these high nickel content alloys contain a large amount of austenite that can tolerate large amounts of hydrogen. This austenite therefore substantially reduces the risk of cold cracking; in addition, they are conventionally welded with nickel based alloys that reduce the risk even further.

Post weld heat treatment is not generally required for the 9%Ni steels; indeed, EN 13445-4 recommends that PWHT should be avoided. The ASME codes, however, specify a PWHT of 552°C to 585°C for both 9%Ni and 5%Ni alloys when thickness exceeds 51mm (2 inches). There are also differences in PWHT requirements in the EN and the ASME specifications for the other types of low temperature steels discussed in this article as tabulated below.

Close control of the PWHT temperature is most important as nickel reduces the lower transformation temperature.

Exceeding the specified temperatures, particularly of the 3.5%Ni and above alloys, may cause the parent metal to transform, resulting in a substantial loss of tensile strength.

One significant problem that is frequently encountered with the nickel steels is that of residual magnetism causing arc blow. This is a particular problem with the 9%Ni steel which can become easily and very strongly magnetised, making it impossible to weld with the arc welding processes. Extreme care needs to be taken during handling, transportation and erection to minimise the effect. Use of alternating current during welding can help overcome some of the difficulties but it may be necessary to degauss the area surrounding the weld.

This article was written by Gene Mathers.