Development of 7% Nickel Steel for LNG Storage Tanks

Abstract

The construction costs of LNG storage tanks are very high, mainly due to the availability and cost of nickel. Most LNG storage tanks built in the modern era use 9% wt nickel steel as the materials of construction because of the materials well documented toughness at cryogenic temperatures. This article looks at advances in cryogenic metallurgy in order to reduce construction costs while maintaining a safe operational environment.

Introduction

The May 2021 Tip of The Month (TOTM) discussed the rationale behind the choice of 9% wt Ni steel for inner tank construction of aboveground LNG storage tanks. This TOTM explores the development of 7% wt Ni steel for the same application with the potential for significant cost savings while maintaining a high level of toughness and strength at LNG temperatures.

The advantages of 9% steel at cryogenic temperatures are well documented and the steel is specified in API 620 Appendix Q as ASTM A553 type 1. The ASME Boiler & Pressure Vessel Code Part II has a chemical requirement of SA-553 Type 1 (equivalent to ANSI A553 Type 1) that requires a nickel content of 8.50% to 9.50% with a tensile strength of 690 MPa to 825 MPa (100,050 to 119,625 psi). In addition, the critical parameter is that the material must have a toughness such that a longitudinal Charpy V-notch test energy shall be not less than 34 Joules (0.0323 BTU) at the specified temperature while a transverse V-notch energy shall be not less than 27Joules (0.0257 BTU). The mechanical properties of SA-553 make it a good option for operation at very low temperatures at low stresses.

There is an opportunity for significant cost savings, though, if the nickel content could be safely reduced. With the price of nickel heading towards US$20,000 per tonne, even a small reduction in nickel content could bring a significant cost reduction in the price of a LNG storage tank. Table 1 below shows the projected cost savings of changing to 7% wt nickel instead of 9% wt nickel for three different sized tanks with a nickel cost of US$17,500/tonne.

Table 1 Nickel cost savings in LNG storage tanks of various sizes (thanks to Dr Jay Rajani)

LNG storage tanks have used 9% nickel steel as the main material for the last fifty years; it has been the industry standard for LNG tank material since it was developed by the International Nickel Company in the USA in 1946.  An evaluation of this steel (Operation Cryogenics) showed that stress relief was unnecessary for 9% nickel steel making it much cheaper to manufacture than contemporary high nickel steels.  The difficulty in using even lower quantities of nickel is in attaining the same degree of metal toughness. Maintaining the required toughness and strength for cryogenic applications has been the primary barrier to further nickel content reduction in the steel for many years.

The nickel content of SA-553 steel gives excellent cryogenic fracture toughness due to retained austenite and the fine microstructure from the nickel content combined with specific heat treatment processes.  In the past, reducing the nickel content and using the same heat treatment resulted in a reduction in cryogenic toughness that is unacceptable in LNG storage applications.

Around 1960 the Nippon Steel and Sumitomo Metal Corporation (NSSMC) initiated a research and development program to investigate nickel content while maintaining cryogenic toughness.  The objective of the development program was to develop a steel that resisted both crack initiation and propagation. The steel production method that was used was TMCP (thermo-mechanical controlled processing).

The processes involved in TMCP are:

• Thermo-Mechanical Rolling (TMR)
  • a technique designed to improve the mechanical properties of materials by controlling the hot-deformation process in a rolling mill
• Accelerated Cooling (AC)
  1. Makes it possible to significantly increase strength properties without lowering impact toughness or cold resistance.
  2. Makes it possible to replace the ferritic-pearlitic structure usually formed in steel after conventional controlled rolling with a fine-grained ferrite-bainite structure having a diminished level of striation.
  3. Makes it possible to attain a prescribed level of strength with lower quantities of carbon and alloying elements. That in turn improves the weldability of the steel.
  4. Reduces the load on the mill because a higher finishing temperature is used than in traditional CR (controlled rolling), allowing an increase in rolling speeds through a reduction in the number of pauses made to cool the slabs on the mill.
• Direct Quenching & Tempering (DQ&T)
  • This approach of direct quenching uses plate recalescence (the surface reheating following accelerated cooling due to the heat flow from its core that is still hot) to promote a direct tempering to the product.

Both TMR and AC had previously been used in shipbuilding steel and line-pipe manufacture with good results.  DQ&T with a low heating temperature was shown to reduce crack propagation and improved mechanical properties.  A further process (Lamellarizing) is used to form reverted austenite, improving impact toughness.

Figures 1 and 2 show the difference in heat treatment from conventional heat treatment to the refined heat treatment employed in the thermo-mechanical control process.

Figure 1 Conventional Heat Treat for 9% wt Ni Steel [1]

Figure 2 TMCP Heat Treat process used in 7% wt Ni Steel [1]

Table 2 below shows the resultant mechanical properties of A553 Type 1 (9% wt Ni steel) and A841 Grade G, Class 9 (TMCP 7% wt Ni steel) that has been manufactured using the TMCP process.  Notice that the tensile strength (TS), the ductility (elongation – El) and, crucially, the Charpy V-notch (IV) energy are all identical.

Table 2 Properties of A553 Type 1 and A841 Grade G, Class 9

Source: https://www.kobelco-welding.jp/education-center/technical-highlight/vol15.html

This variant of nickel steel has been approved as a Japanese standard under JIS G 3127, SL7N 590 and has been used successfully in LNG tank construction in Japan and the Middle East.  The second addendum to API 620 twelfth edition Annex Q (2018) now allows the use of A553 Type III (reference ASME Code Case 2842 – specifically for pressure vessels) or A841 Grade G, Class 9 as a standard material for product temperature applications down to -325ºF (-198ºC).  The use of A841 Grade G, Class 9 requires that SA-841 Supplementary Requirement S64 has been satisfied.  S64 is defined in ASME Boiler & Pressure Vessel Code Part II as follows:

S64.1 Except for the TMR-I-T process, the plates shall be cooled directly after rolling without being allowed to cool below 1025°F [550°C]. Quenching hardening shall be initiated from a temperature within the range from 1025 to 1490°F [550 to 810°C].

S64.2 Subsequent to quenching, the plates shall be tempered within the range from 1030 to 1155°F [555 to 625°C], holding at that temperature for a minimum of 30 min/in. [1.2 min/mm] of thickness but for not less than 15 min, and then cooling at a rate of not less than 300°F/h [165°C/h], either in air or by quenching in water, to ambient temperature.

S64.3 Prior to the tempering treatment, the plates may be subjected to an intermediate heat treatment (Note S64.1) consisting of heating to a temperature in the range from 1185 to 1310°F [640 to 710°C], holding at that temperature for a minimum of 1 hr/in. [2.4 min/mm] of thickness, but in no case less than 15 min, and then water-quenching to below 300°F [150°C] in the case of plate thicknesses of more than 5⁄8 in. [16 mm]; or cooling in air or water-quenching in the case of plate thickness of 5⁄8 in. [16 mm] and under.

NOTE S64.1—The intermediate heat treatment is for the purpose of enhancing elongation and notch-toughness and for reducing susceptibility to strain-aging embrittlement and temper embrittlement. It may be performed at the option of the material manufacturer or may be specified by the purchaser.

S64.4 Heat treatment temperatures and times shall be reported in accordance with Section 19 of Specification A20/A20M.

The potential for cost savings runs into hundreds of thousands of US dollars for each LNG tank meaning this material should be considered as a valid alternative to 9% nickel steel for future LNG inner tank construction.

References

1. Hiroshi Nagami et al., Development and realization of large scale LNG storage tank applying 7% Nickel steel plate, Kuala Lumpur World Gas Conference 2012
2. Hitoshi Furuya et al., DEVELOPMENT OF LOW-NICKEL STEEL FOR LNG STORAGE TANKS, Nippon Steel & Sumitomo Metal Corporation
3. Takayuki Kagaya et al., New Steel Plate for LNG Storage Tank, Nippon Steel & Sumitomo Metal Corporation technical report No. 110 September 2015.
4. Antonio Augusto Gorni et al., Accelerated Cooling of Steel Plates: The Time Has Come, Journal of ASTM International, Vol. 5, No. 8, Paper ID JAI101777,  Available online at www.astm.org
5. API 620 Design and Construction of Large, Welded, Low-Pressure Storage Tanks, Twelfth Edition, October 2013, Addendum 2, April 2018
6. ASME Boiler and Pressure Vessel Code, Section II, Part A
7. ASME Code Cases July1, 2019.  CC2842