Use of Low alloy steels at high temps

[Talha Aamir]

Hi fellows,

I wanted to know why low alloys are used in high temperature service and how they resist damage mechanisms like 475C embrittlement, HTHA, Graphitization. I guidance regarding reasons behind their selection other than cost savings.

Thanks.

[Mohd Siraj]

this is not fully answer to your very elaborative question, but in short,

1. Low-alloy steels contain alloy element Cr, Mo, V, Ni

2. Low-alloy steels has a balanced compositions of above alloy element to resist  475C embrittlement, HTHA, Graphitization. 

[Amol Betkar]

Dear Aamir,

Low alloys are often selected for high-temperature service due to their unique combination of properties that provide resistance to various damage mechanisms such as 475°C embrittlement, high-temperature hydrogen attack (HTHA), and graphitization. Here's a detailed explanation of why they are used and how they resist these specific damage mechanisms:

Reasons for Selection Beyond Cost Savings

1. Mechanical Strength at Elevated Temperatures:

   • Low alloy steels, especially those containing elements like chromium (Cr) and molybdenum (Mo), maintain good mechanical strength at elevated temperatures. This makes them suitable for applications where structural integrity is crucial at high temperatures.

2. Resistance to Oxidation and Corrosion:

   • Additions of Cr and Mo enhance the oxidation and corrosion resistance of low alloy steels. Chromium forms a stable oxide layer on the surface, protecting the material from further oxidation. This is particularly important in high-temperature environments where oxidation can be a major issue.

3. Control of Embrittlement Mechanisms:

   • 475°C Embrittlement: This phenomenon primarily affects ferritic stainless steels but can also impact low alloy steels with high Cr content. The embrittlement is due to the formation of a Cr-rich α' phase. Low alloy steels are designed to balance the Cr content to minimize this risk, often by maintaining it at levels that do not favor significant α' phase formation.
   • High-Temperature Hydrogen Attack (HTHA): HTHA occurs when hydrogen diffuses into the steel and reacts with carbon to form methane, which can cause internal cracking. Low alloy steels resist HTHA through the presence of carbide-forming elements such as Mo, which stabilize carbides and reduce the activity of carbon available to react with hydrogen.
   • Graphitization: This damage mechanism involves the formation of graphite nodules from the decomposition of carbides at high temperatures, leading to a loss of strength. Alloying elements like Mo and Cr help in stabilizing carbides, making low alloy steels less susceptible to graphitization.

4. Heat Treatment Flexibility:

   • Low alloy steels can be heat treated to achieve a range of mechanical properties tailored to specific high-temperature applications. For example, normalizing and tempering can improve toughness and strength, while annealing can enhance ductility and reduce internal stresses.

5. Compatibility with High-Temperature Fabrication Processes:

   • Low alloy steels are generally more compatible with welding and other fabrication processes at high temperatures. The presence of alloying elements helps in reducing the risk of weld cracking and enhances the overall weldability of the material.

Resistance to Specific Damage Mechanisms

1. 475°C Embrittlement:

   • Control of Cr content and the use of appropriate heat treatments reduce the risk of 475°C embrittlement in low alloy steels. The formation of the Cr-rich α' phase is minimized, maintaining ductility and toughness.

2. High-Temperature Hydrogen Attack (HTHA):

   • Mo and other carbide-stabilizing elements reduce the mobility of carbon, limiting methane formation and internal cracking. The material's resistance to HTHA is enhanced through controlled alloy composition and microstructural stability.

3. Graphitization:

   • Alloying elements like Mo and Cr help in stabilizing carbides against decomposition into graphite. This ensures that the steel retains its mechanical properties over prolonged exposure to high temperatures.

Conclusion

The selection of low alloy steels for high-temperature service is driven by their superior mechanical properties, oxidation and corrosion resistance, and the ability to resist specific high-temperature damage mechanisms. These steels are engineered through careful alloying and heat treatment to perform reliably in demanding high-temperature environments, making them an excellent choice for such applications beyond just cost considerations.

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Best regards,

Amol K.B

B.E(Mech), CSWIP 3.2.2, NACE CIP 1 & ASNT LII(4M)

[Kannayeram Gnanapandithan]

475 may not happen in low alloy steel.

Sigma phase formation will happen when --if %Cr-16%C/%Ni,     Less than 1.7, No sigma phase,  if it is more than 1.7 , Sigma phase

THANKS & BEST REGARDS,

KG.PANDITHAN, BE, IWE,  CSWIP 3.1,

ISO 9712 Level 2 in VT,

ASNT-Level II in PT,MT,RT & UT,

LA ISO 9001-2015,

International Welding Engineer. 

CONSULTANT-WELDING & QUALITY

Mobile no: +919940739349

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