Technical requirements for petrochemical reactor steels have proliferated in the last decade. The need to increase economic benefits together with higher operating temperatures and pressures are leading to the construction of higher capacity reactors with thicker walls.

Also, more severe requirements in these steel specifications leave the steelmaker little scope to find a good balance between all the influencing parameters, in order to achieve the best possible steel design.  

These two points can lead to a situation where the material’s physical limits are reached. It is not only the heat treatment, but also the chemical composition and thickness of the plate that strongly influence the steel’s mechanical properties.  

By taking the example of carbon manganese steel SA516-70 edition 2017 [2], we will look at how the microstructure is modified and thus the main mechanical properties.  

It may be noted that since 2007, it is permitted to have a maximum manganese content of 1.50% as against 1.20% so long as the maximum carbon content is also reduced. However, this cannot be implemented if the pressure equipment has to be constructed according to the 2004 edition addendum 2006 or previous editions. 

THE HOLLOMON PARAMETER (HP) CONCEPT

The Hollomon parameter (HP) [1] is used to describe the various heat treatments applied to a steel plate, especially tempering and stress relief treatments as well as their combination (see figure 1). This parameter is useful as it combines the various influencing factors of heat treatment into a single value.  

Thus different heat treatments having an identical HP will have an identical effect on the material’s metallurgy − the higher the HP, the more significant the metallurgical effect. 

INFLUENCE OF THE HP ON THE PLATE’S MECHANICAL PROPERTIES  

Figure 2 shows the evolution of yield strength and tensile strength for a typical chemical composition and for plates with thicknesses between 30 and 50 mm, against the HP (N representing the normalised state). The blue-shaded band represents yield strength values calculated with a confidence interval of ±2.3σ. The red-shaded band represents tensile strength values calculated with a confidence interval of ±2.3σ. The continuous red and blue lines represent the minimum and maximum requirements set by standard SA516-70 ed. 2017 [2].  

We can see that yield strength and tensile strength are affected by the heat treatment: the harsher the heat treatment, the more the material’s yield strength and tensile strength decrease.  

Figure 3 shows the evolution of toughness against HP. The coloured lines represent, for various test temperatures, the lower toughness limits calculated for a typical chemical composition and for plates with thicknesses between 30 and 50 mm. 99% of the results are thus found above this limit. The two horizontal black lines represent typical minimum values 25 J average (17 J lowest single value).  

We can see that the toughness is strongly affected by the heat treatment − the harsher the heat treatment, the more the material’s toughness decreases.  

Therefore, by keeping the chemical composition constant whatever the HP, the requirements set by a standard or customer may no longer be complied with. To counter this effect, the steel’s carbon content can be adjusted. Figures 4 and 5 are curves of the expected values of tensile strength and toughness at –29°C for plate thickness of 50 mm and typical chemical composition by only varying the carbon content and the HP.  

As figures 4 and 5 show, the carbon content has to be increased to ensure the minimum tensile strength requirement, and it has to be decreased to improve toughness. For certain carbon contents, and according to the heat treatment applied, it is no longer possible to ensure the product’s mechanical requirements without employing other solutions. These other solutions will be examined below.  

INFLUENCE OF PLATE THICKNESS ON MECHANICAL PROPERTIES  

Figure 6 shows the evolution of yield strength and tensile strength, for a typical chemical composition and for normalised plates, against thickness. The blue-shaded band represents yield strength values calculated with a confidence interval of ±2.3σ. The red-shaded band represents tensile strength values calculated with a confidence interval of ±2.3σ. The continuous red and blue lines represent the minimum and maximum requirements set by standard SA516-70 ed. 2017 [2].  

When plate thickness increases, the material’s yield strength and tensile strength decrease for a constant chemical composition. Therefore, from a certain thickness, it is no longer possible to ensure the minimum tensile strength requirements set by the standard by keeping the steel’s same chemical composition. Normally the carbon content has to be increased.  

Figure 7 shows the evolution of toughness against plate thickness. The coloured lines represent, for various test temperatures, the lower toughness limits calculated for a typical chemical composition. 99% of the results are thus found above this limit. The two horizontal black lines represent typical minimum values 25 J average (17 J lowest single value).

For low plate thicknesses, the cooling rate (in air) is relatively high and can produce a ferritic-perlitic microstructure containing bainite. This negatively affects toughness in the untempered state. Therefore the chemical composition and/or heat treatment requires adjustment.  

Above a certain thickness where toughness has reached a maximum, the material’s toughness decreases with increased plate thickness, all other parameters remaining constant. This phenomenon is mainly due to increased grain size of the microstructure since the plate’s cooling rate decreases.  

Therefore, to ensure the minimum requirements of the standard (or customer), for a given test temperature and thickness, it is necessary to modify the steel’s chemical composition and/or heat treatment.  

Let us look again at the solution of varying the carbon content to alleviate the negative effect of increasing the product’s thickness on the material’s mechanical properties. Figures 8 and 9 show the expected values of tensile strength and toughness at –29°C for a typical chemical composition of plate in normalised condition by varying the carbon content and plate thickness. As in the previous case, it appears necessary both for the carbon content to be increased to ensure the minimum tensile strength requirement, and to be reduced to improve toughness.  

Finally for certain combinations “product thickness – heat treatment – toughness test temperature – standard or customer specification”, it can easily be seen that the single carbon content variation is no longer sufficient. Furthermore, carbon negatively affects certain steel forming properties, especially its suitability for oxycutting or its weldability. Finally, carbon comes directly into the calculation of the carbon equivalent which is often limited by the standards or customer specifications.

Therefore other methods have to be used to adjust the mechanical properties of the plates.  However, these have to comply with the standard or be accepted by the customer.  

ALLOYING

A reduction of carbon in the steel can be offset by adjusting the steel's chemistry using different alloying elements such as chromium, molybdenum, copper, nickel and/or niobium. Thus the alloying elements, by their effect, contribute directly to improving the steel's toughness, or allow to reduce the carbon content without reducing tensile strength and so indirectly improving the product's toughness. 

Figure 10 shows the effect of alloying on the mechanical properties in the form of zones.  These zones are functions of the thickness and heat treatment depending on whether a "CMn" concept or a "CMn + alloying" concept is used. These zones represent (t; HP) combinations for which a plate can be produced according to standard SA516-70 ed. 2017 with minimum toughness in the transverse direction at quarter thickness of 25 J average (17 J lowest single value) at –29°C.  

However, this concept also has limits and certain "t; HP" combinations cannot be reached. Standards and specifications restrict the alloying element content and these elements influence the carbon equivalent (the most commonly used formula is:  CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15). Furthermore, it should not be forgotten that the production cost of an alloyed plate is significantly higher than that of a carbon-manganese plate.

Therefore, it is necessary to explore other avenues, for example by directly adjusting the parameters of the heavy plate production process. 

ACCELERATED COOLING FOLLOWED BY TEMPERING

This solution consists in the accelerated cooling of the plates in order to produce a microstructure having a finer grain size and comprised of bainite and martensite.  Nevertheless, to achieve good toughness on these plates, they have to receive a heat treatment (tempering) after quenching.

However, certain standards or specifications do not permit this special treatment. Yet, according to standard SA516, if approved by the purchaser, for improvement of the toughness, it is possible to apply cooling rates faster than those obtained by cooling in air. This accelerated cooling must be followed by tempering in the temperature range 595 to 705 °C. The plate's delivery condition is no longer "N" but "N + AC + T".  

The goal is to produce the same microstructure as thin plate for thick plate. Grain size directly influences toughness. By acting on this parameter it is possible to satisfy the requirements in terms of toughness, while keeping enough carbon and alloying element content to reach the required levels of yield strength and tensile strength.  

Figure 11 shows typical plate microstructures obtained by means of these two types of process.  

Figure 12, like figure 10, shows the effect of the "N + AC + T" treatment on the mechanical properties. Here the zones also represent (t; HP) combinations for which a plate can be produced according to standard SA516-70 ed. 2017, with minimum toughness in the transverse direction at quarter thickness of 25 (17) J at –29°C, by the production method used.  

By implementing this type of process (when the standard or the customer specification permits) the steelmaker can accept more severe requirements in terms of the (t; HP) combination or in terms of a pure requirement with constant (t; HP) combination. Other properties can also be enhanced thanks this type of process. Nevertheless the surface hardness is slightly increased by using N+AC+T instead of N.

CONCLUSION  

By means of various examples we have seen that the mechanical properties of a heavy plate depend on many parameters. Not only the chemistry employed in preparing the steel in the steel plant, but also the thickness of the finished product and the type of process implemented strongly affect the characteristics of the finished product. Thus requirements that are too severe compared with those defined in the standard, especially when they only take account of part of the influencing parameters or when they are applied uniformly to an entire range of thicknesses, limit the steelmaker's options to adjust the mechanical properties of heavy plate. For highly demanding specifications, as increasingly required by customers, it is therefore important to hold detail discussions with the steelmaker before ordering plate.