How to Weld 9% Nickel Steel


How to Weld 9% Nickel Steel

The low-temperature limit for the safe application of unalloyed steel is around -29°C (e.g. ASTM A283 plates). Heat treatments like normalizing could lower this limit down to -50°C (e.g. ASTM A516) but without alloying the steel, that’s as far as you can go.

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Nine percent nickel steel is often the most economic material of construction for larger-size shop-fabricated vessels for service down to -320°F where high strength is required. The nickel steels do not exhibit a “stainless” behavior and can oxidize or rust in many environments. For this reason, the steels are not suitable for the storage of gases such as argon where contamination can be a concern, e.g., semiconductor applications.

A larger application of 9% nickel steel is for large LNG field storage tanks and for containment vessels on LNG tankers. Welding practices for field construction may be similar to shop procedures, although fabricators often develop proprietary welding procedures for certain welds. The applicable codes for field construction are usually different from those used for shop fabricated vessels.

What is 9% Nickel Steel & its importance?

9% Nickel Steel’s unique properties come from its crystal structure. Theoretically, it can be shown that materials exhibit ductile behavior if their crystal structure exhibits 5 distinct “slip systems” – meaning that the atoms forming the solid can be easily dislocated in 5 different ways.

Typical steel has only 2 or 3 slip systems at low temperatures. However, Nickel exhibits a large number of slip systems, which persist even at low temperatures. In effect Nickel never undergoes a ductile-brittle transformation. What’s more, these slip systems persist even when Nickel is alloyed with other metals. The magic number turns out to be 9%: when steel is made with this proportion of Nickel, the metal contains a small proportion of “austenitic” steel grains which are inherently ductile. Though these grains make up only around 4% of the steel’s volume, they are enough to imbue it with a resistance to the weakening effects of low temperatures, while retaining as much toughness as possible.

Material specification for 9% Ni Steel

ASME P Number: P-No. P11A Group 1

Two material specifications widely used for 9% nickel steel plates are:
1. ASTM SA-353/SA-353M, Specification for Pressure Vessel Plates, Alloy Steel, 9% Nickel, Double-Normalized and tempered.

2. ASTM SA-553/SA-553M, Specification for Pressure Vessel Plates, Alloy Steel, Quenched and Tempered 8 and 9%Nickel.

Material specifications for 9% Nickel plate (UNS K81340) intended for cryogenic pressure vessels are ASTM A353 (double normalized); ASTM A553 Type I (quenched and tempered); ASME Section II, Part A, SA353 and SA553; and API Standard 620 Appendix Q, A353 and A553.
Rules for design, fabrication, and testing of pressure vessels are given by the ASME Code Section VIII, Divisions 1 and 2 (including applicable Code Cases 2214, 2335, and 2345), Section III Class 3 Components, and API Standard 620 Appendix Q.

Chemical Properties of 9% Nickel Steel

How to Weld 9% Nickel Steel

Mechanical Properties of 9% Ni Steel

Due to its high strength and good toughness at service temperatures down to -196°C, the 9% Nickel steel is widely used in the fabrication and construction of LNG tanks and cryogenic vessels. Its lowest grade (i.e. A353) has a high yield strength (515 MPa/75 ksi) which is considerably higher than its austenitic stainless steel competition like SS304 grade (205 MPa/30 ksi) and could reduce the thickness and weight of the plates down to 40%.

How to Weld 9% Nickel Steel

API & ASME Impact requirements for 9% Ni Steel

How to Weld 9% Nickel Steel

Microstructure of 9% Nickel Steel

Generally, 9% nickel steel pressure vessel plates may be furnished in double-normalized and tempered conditions (ASTM A353), quenched and tempered conditions (ASTM A553), or direct-quenched and tempered condition (ASTM A844). The microstructure of 9% nickel steel after heat treatment consists of fine tempered-martensite and appreciable retained austenite (about 5% to 15% structure size) which contributes to the excellent cryogenic toughness of this steel. Since the alloying addition of approximately 9% nickel also suppresses the formation of ferrite/pearlite high-temp transformation phases, a relatively higher strength is also attained.

How to Weld 9% Nickel Steel

Weldability Concerns of 9% Nickel Steel

While the weldability of these alloys is not an issue, achieving satisfactory results requires careful selection of welding process & consumables and also close control of welding parameters during both qualification and fabrication stages. The most important considerations for welding of 9% Nickel steel are as following:

  • Always consult with the filler metal manufacturer before the preparation of your WPS and during the welding operations.
  • Use Ni-base filler metals – There are two main options here; (1) austenitic stainless steel filler metals and (2) Ni-base consumables. In general, Ni-base filler metals produce welds with better mechanical properties and have proved to be more cost-effective.
  • The thermal expansion coefficient (CoE) of nickel-based weld metals closely matches the 9% nickel steel itself; this value is 50% higher for austenitic stainless steel weld metal with an associated higher risk of thermal fatigue in cycling operations.
  • Among available Ni-base filler metals, NiCrMo-6 is currently the most popular choice. In general, if you require higher weld tensile strength, NiCrMo (Inconel family) consumables are preferred. Fabricators also use NiMoCr (Hastelloy family) filler metals for their higher ductility and their resistance to hot cracking (due to higher Mo content). This type of filler metal is mostly used in high-speed SAW welding operations.
  • Demagnetize the plates before welding and use AC current for welding. 9% Nickel steel could easily be magnetized and cause welding arc blow.
  • Keep heat input low; the maximum inter-pass temperature should be 150°C and heat input shall not exceed 2 & 3 KJ/mm for SMAW & SAW respectively.
  • Do not use PWHT unless it is absolutely necessary (i.e. required by the code). The application of PWHT on this alloy has a negative effect on tensile properties.
  • Keep it clean – Ni-base welds are susceptible to hot cracking and require a high level of cleanliness.
  •  Try to avoid highly constrained joints which increase the likelihood of centerline cracking in the welds deposited by fully austenitic Ni-base consumables.

Factors Affecting Low-Temperature Weld Properties

  1. Low ferrite. It is well known that ferrite in austenitic stainless steel welds, e.g., 4 to 8 FN, is a strong deterrent to micro-fissures and cracks. However, ferrite over 3 to 4 FN reduces the low-temperature impact strength, while ferrite as low as possible, or even strongly austenitic, provides higher impact strength. A safe compromise at about 2 FN yields the highest possible impact strength and good resistance to hot cracking. The shape of the weld metal ferrite also affects impact strength, however, this is a factor that is difficult for production shops to control.
  2. Low carbon. Low carbon, preferably in the range of 0.03% or less, provides better weld metal toughness.
  3. Low nitrogen. Nitrogen increases the tensile and yield strength of stainless steel welds but decreases the low-temperature toughness. One study suggests that the weld deposit of Type 304 and 316L is preferably held below 0.05% nitrogen.
  4. Higher nickel. Higher nickel, within the permissible specification range, has been found to increase weld metal toughness. Some investigations indicate that weld toughness is significantly increased when nickel is increased from 1 to 20%, however, the higher-nickel stainless filler has not been commercialized.
  5. Lime-type electrodes. Shielded metal arc welding (SMAW) lime-type electrodes have higher low-temperature toughness than titania-type electrodes and are largely the standard electrode for cryogenic components.
  6. Low weld metal inclusion content. It has been shown that in Type 316L welds, a low inclusion content increases low-temperature toughness. The welding process and welding procedure can have a strong influence on the inclusion content. For example, fabricators usually find the gas metal arc welding (GMAW) process to be more reliable than the submerged arc welding (SAW) process in meeting the production weld impact test required at temperatures below -325°F. Since weld inclusions are usually some type of oxide, low oxygen levels in the shielding gas aid in keeping inclusions to a minimum.

Preheat for 9% Nickel steel

Preheat Joints in 9% Nickel steel made with austenitic weld metals are relatively immune to cold cracking difficulties. Nevertheless, it is suggested that plate over 1in.(25mm) in thickness be preheated to about 100ºF(35ºC) and that lighter plates not be welded below the dew point. Section VIII Code Case 2214 provides additional information on preheat requirements in special situations.

Post Heating (PWHT) for 9% Ni Steel

Post-Weld Heat Treatment Except as provided by the fabrication requirements of ULT-79, the ASME Code requires no post-weld heat treatment for 9% Nickel up to 2 in. (51 mm)inclusive in thickness. See AF630.1 (Division 2) and UHT56 (Division 1). Where post-weld heat treatment is performed, it is necessary to control the temperature within the range of 1025-1085ºF (551-583ºC) but not over the tempering temperature and to cool at a rate not less than 300ºF(167ºC) per hour to avoid a possible reduction in notch toughness of the steel.

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