The Importance of Restricting Ni+Mn in P91 and P92 Welding

In the world of metallurgy and welding, certain alloy steels are revered for their exceptional high-temperature properties, among them being P91 and P92. These alloys find extensive use in power plants, industrial boilers, and other high-temperature applications. However, there’s an interesting restriction imposed on these materials: the combined content of Nickel (Ni) and Manganese (Mn) (Ni+Mn in P91 and P92) must not exceed 1.2% or 1%. In this article, we’ll delve into why this restriction exists and its critical role in the welding process.

The Role of Postweld Heat Treatment (PWHT):

Before we explore the Ni+Mn restriction, let’s understand the importance of Postweld Heat Treatment (PWHT) in welding P91 and P92 steels. PWHT is a crucial step in the welding process, where the welded material is heated to a specific temperature and then slowly cooled down. This process helps relieve stress, improve mechanical properties, and refine the microstructure of the welded joint.

PWHT is one of the most important factors in producing satisfactory weld of P91 material. Regardless of the thickness or diameter preheating and PWHT are mandatory to reduce hardness and to restore ductility in the weld and HAZ. Selection of P91 steel PWHT temperature range is very tricky as we have to follow construction code and also depends & affected by percentage of Mn+Ni as it lowers the lower critical transformation temperature. Selection of proper soaking temperature is very important in case of P91 material. If the PWHT temperature is too low, the welds joints lead to insufficient toughness and high hardness due to the unsatisfactory tempering effect and if the PWHT temperature is too high, the tensile strength at ambient and elevated temperatures becomes inadequate due to over tempering effect.

PWHT temperature as per various codes and standards:

705-785°C (as per Table PW-39-5, ASME Section 1)
705°C Minimum and for Max. temp. refer below (as per Table UCS 56-11, ASME Section VIII Div.1)

For welds made with matching Grade 91 filler metal (e.g., AWS A.5.5 E90xx-B91, ISO EN CrMo91), the maximum holding temperature shall be determined as follows:

If the Ni+Mn content of the filler metal≤1.0%, the maximum PWHT temperature shall be 790°C.
If the Ni+Mn content of the filler metal>1.0% but ≤ 1.2%, the max. PWHT temperature shall be 780°C.

If the Ni+Mn content of the filler metal>1.2%, the maximum PWHT temperature shall be at least 10°C below the lower critical transformation temperature (Ac1)

705-775°C (Table 132.1.1-1, ASME B31.1 & Table 331.1.1 of ASME B31.3)

Recommended PWHT temperature for P91 Material are 740 to 760°C

The Influence of Mn and Ni on Ac1 Temperature:

One of the key factors in determining the success of PWHT is the Ac1 temperature, which refers to the lower transformation temperature. In the context of P91 and P92 welding, this temperature is relatively low. Here’s where the restriction on the combined content of Manganese and Nickel comes into play.

The Mn+Ni Restriction Explained:

Manganese and Nickel are alloying elements commonly found in P91 and P92 steels. However, the presence of these elements in excess can have an undesirable effect on the Ac1 temperature. When Mn and Ni are present in higher concentrations, they tend to lower the Ac1 temperature. This can be problematic during PWHT because it means that the PWHT temperature might approach or even exceed the Ac1 temperature.

The Risk of Partial Transformation:

Why is exceeding the Ac1 temperature during PWHT a concern? It’s because it can lead to partial transformation of the microstructure. In simpler terms, the steel might undergo structural changes that are not desirable for its intended high-temperature applications. This partial transformation can compromise the mechanical properties and overall performance of the welded joint.

The Solution: Restricting Mn+Ni Content:

To ensure that PWHT remains effective and doesn’t inadvertently induce partial transformation of the microstructure, a restriction is imposed on the combined content of Manganese and Nickel in P91 and P92 steels. By limiting the Mn+Ni content to 1.2% or below (as specified in Table 2, note g), the PWHT temperature can be maintained at a safe distance below the Ac1 temperature. This prevents any unwanted structural changes and ensures that the welded joint retains its intended high-temperature properties.

Conclusion:

In the world of welding and metallurgy, understanding the intricacies of alloy composition and heat treatment processes is paramount. The restriction on the combined content of Manganese and Nickel in P91 and P92 steels may seem like a minor detail, but it plays a vital role in maintaining the integrity and performance of high-temperature applications. By adhering to this restriction, welders and engineers can ensure that the welded joints in power plants and industrial boilers remain strong, reliable, and capable of withstanding extreme heat and pressure.

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