Highlighting the advantages of TMCP steel plates for the construction of storage tanks
There have been more than four decades of experience in the successful application of thermo-mechanically hot rolled heavy plates in the fields of offshore oil and gas platforms, offshore wind power, shipbuilding, steel construction and most popularly, for the construction of large diameter oil and gas pipelines based on the API standards.
Considering the construction of pressure vessels and storage tanks, there is a contrary situation. In spite of the evident advantages of thermo-mechanical controlled processing (TMCP) steels, their usage in this sector is rather uncommon.
For pressure vessels comprising heads, where a new quality heat treatment after processing becomes mandatory or for pressure vessels with high wall thicknesses, which need a considerable post weld heat treatment (PWHT), there are clear arguments against these steels.
But regarding the fabrication of large storage tanks for hydrocarbon products and ammonia, the application of thermo-mechanically rolled steels offers considerable technical and economic benefit for the manufacturer as well as the end user. This benefit increases if these steels are applied in sour service conditions, where the risk of hydrogen induced cracking (HIC) is present.
As the pressure vessel industry is highly regulated, it is very important that the use of TMCP steels is permitted by the applying codes. Consequently, Dillinger experts reviewed the globally established construction codes such as ASME BPVC, API 620, EN13445, EN14620 and so on, (Table 1) with the result, that all of them allow the application of thermo-mechanically rolled steels up to about 40mm in thickness.
The basics TMCP steels
Steels made by TMCP are not only characterised by high strength and excellent toughness, but also by outstanding weldability.
Conventionally it has been impossible to obtain these characteristics simultaneously and one had to be chosen at the expense of the other. With regard to normalised steels, higher strength could only be achieved by increasing the content of alloying elements, which in turn has a negative impact on the weldability. The situation is different with TMCP steels. They achieve their strength properties by a combination of a process-optimised, very lean chemistry (Table 2) and a special rolling technique (Fig. 1), resulting in a unique, fine grained microstructure compared to normalised steels (Fig. 2).
Strength levels and alloying content
The relationship between achievable strength levels and required alloying contents is clearly shown in Fig. 3.
The content of alloying elements is described by the carbon equivalent. It can be seen that it is possible to achieve the same strength level with a considerable reduction in alloying elements for TMCP compared to normalised steels. This is based on the Hall-Petch relationship, which provides an improvement of strength and also toughness if the grain size of a microstructure gets reduced.
The combination of a fine-grained microstructure and a very lean chemical analysis, which is typical for TMCP steels, leads to several advantages when it comes to further processing of the plates.
Subsequently the advantages for different production steps are illustrated.
Advantages of TMCP plates during fabrication
The first step during fabrication of storage tanks is the cutting and weld edge preparation of the delivered plates. Due to their lean chemical analysis, TMCP steels show only a very low susceptibility to hardness increase. This means there is no need for any preheating before the thermal cutting process.
The second step during fabrication of storage tanks is the bending of the shell plates. Like all other production steps (especially welding), bending shifts the impact transition temperature of the material to higher temperatures. This is equivalent to a reduction of the material’s toughness. TMCP plates offer a significantly higher toughness level than normalised steels (Fig. 4).
The advantage for the fabricator is a safer production process arising from the higher toughness reserves and lower susceptibility to brittle fracture.
The welding advantage
The most challenging process during the fabrication of storage tanks is the welding of the bended shell segments. Generally this production step consists of three separate processes, which are preheating, welding and PWHT.
For normalised material it is usually mandatory to carry out a preheating to avoid excessive hardening and cold cracking after welding. Regarding the European codes EN1011-2 gives the following formula to calculate the necessary preheating temperature (See Formula 1).
Tp: Preheating temperature [°C]
CET: Carbon equivalent [%] (CET=C+((Mn+Mo))/10+((Cr+Cu))/20+Ni/40)
d: plate thickness [mm]
HD: Hydrogen content [ml/100g]
Q: Heat input [kJ/mm]
The advantage of TMCP steels becomes clear when the formula is plotted into a diagram for different steels (Fig. 5).
The diagram shows that a thermo-mechanically rolled steel with a yield strength of 355 MPa, such as SA841-A-1 or P355ML2 (which are equivalent to normalised SA516-70 or SA537-1) can be easily welded with no preheating up to 40mm plate thickness, while the normalised grades would need a preheating in the range of 60-100°C.
Furthermore, a TMCP steel with a yield strength of 420 MPa, such as SA841-B-2 or P420ML2, which are equivalent to quenched and tempered SA537-2, might also be welded with no preheating up to 40mm plate thickness, if the hydrogen and heat input are carefully controlled.
The benefit for the fabricator is that no preheating before welding becomes necessary, which results in a faster fabrication, less energy consumption and lower costs. This advantage of TMCP in comparison to normalised steels arises from their lean chemistry, which is represented by the CET value in the given equation.
Referring to the ASME BPVC, the same advantage of TMCP steels regarding preheating is included.
Based on table 6.8 of ASME BPVC VIII Div. 2 for P-No.1, Group 1, 2, 3 steels, which are normalised steels, a PWHT plate thicknesses between 32mm and 38mm becomes mandatory, unless a preheating of 95°C has been applied. For the TMCP grades SA841-A-1 and SA841-B-2 this preheating is not necessary, if their carbon equivalent according to S77 of ASTM A841 does not exceed 0.40 %.
Concerning the welding process itself, there are two main requirements relating to the plate material that have to be fulfilled.
On the one hand, the impact values in the heat-affected zone have to exceed the required values from the codes. On the other hand, the hardness values have to stay under a defined value.
Due to their lower contents of carbon, alloying and microalloying elements, TMCP plates show superior toughness even with high heat inputs and reduced hardness values in the heat affected zones compared to normalized steels (Figure 6 and Figure 7).
Subsequently TMCP steels offer an increased working range for the welding process, resulting in an increased safety to fulfil the requirements of the code, especially due to the fact that storage tanks are fabricated on site and not in a workshop.
One disadvantage of TMCP steels is their limited resistance against PWHT with holding temperatures above 600°C. However, for most storage tanks no PWHT is required by the codes and if required the codes offer lower temperatures with longer holding times. In general the best idea is to agree about the parameters of PWHT with the steel supplier at the time of the inquiry.
TMCP steels in sour service
Even if the stored gas or liquid causes the risk of sour service-related HIC, the application of TMCP plates has advantages. For normalised plates a stress relieving of the plate or a PWHT of the whole vessel even under 38mm is necessary to achieve HIC resistance, verified in accordance with NACE TM 0284. For TMCP plates this additional heat treatment is not required.
After investigating a large number of plate enquiries, studying the requirements of different construction codes and having in mind the benefits of TMCP steels, Dillinger developed a tailor-made TMCP steel branded Di-Tank for the construction of storage tanks.
Di-Tank is available with 355 MPa and 415 MPa yield strength in accordance to different construction codes, offering improved toughness properties and a very low carbon equivalent compared to grades listed in Table 1. Di-Tank 355 is also available with improved resistance to hydrogen induced cracking, offering a CLR ≤ 10 %, CTR ≤ 3 % and CSR ≤ 1 %, verified in accordance with NACE TM 0284.
TMCP plates offer various advantages compared to equivalent normalised plates and are furthermore in accordance with the construction codes for storage tanks.
During recent years some projects using TMCP steels have been realised, achieving overall cost savings of approximately 10%.
The fact that TMCP steels are uncommon for the application of storage tanks derives from the experience-based approach by the specification writers regarding the selection of materials.
Looking at the worldwide growing energy demand and the need for storage along the whole oil and gas value chain, there will be many future opportunities to profit from the use of thermo-mechanically rolled heavy plates.
Dr Peter Flüß, Joerg Maffert and Philipp Schirra are with Dillinger.