Understanding Metallurgy for Valve Applications

Material selection for valves is a complex process that must consider temperature, pressure, corrosion and wear. A comprehensive understanding of metallurgical principles enables engineers and manufacturers to make informed decisions, ensuring valves perform reliably in demanding environments while reducing operational costs and improving safety.

What is metallurgy? 

Metallurgy is the science and engineering of metals, focusing on their extraction, processing and transformation into functional components. It examines how composition, structure and processing methods affect mechanical and chemical properties such as strength, toughness, corrosion resistance and thermal stability (see an example in Figure 1). 

            
In valve applications, industry standards such as ASME, API and ASTM define material requirements, pressure ratings and temperature limits to ensure safety and reliability. Metallurgy directly influences these ratings through alloy composition, heat treatment, and microstructural control. 

 

 

 

 

 

 

 

 

 

 

 

Role of heat treatment and crystal structures

Heat treatment processes refine material microstructures to enhance strength, hardness, toughness and corrosion resistance, ensuring optimal performance in industrial applications. The table below summarizes key heat treatment processes. Each process is crucial in modifying the material’s phase and structure to achieve specific characteristics. Solution annealing and precipitation hardening, for instance, are used to optimize corrosion resistance and strength in specific alloys like austenitic stainless steels and nickel-based alloys. By controlling the heat treatment parameters, engineers can tailor materials to meet the demands of high-stress and corrosive environments.

Heat Treatment Process	Material Type	Microstructure Phase/Formed	Effect on Material Properties
Austenitization	Carbon and alloy steels	Austenitic phase (FCC)	Prepare  for hardening, enhances ductility
Quenching	Austenitized steel	Martensite (BCT)	High hardness, but increased brittleness due to trapped carbon
Tempering	Martensite	Refined martensite (BCT)	Reduces brittleness while maintaining hardness
Normalizing	Carbon and low-alloy steels	Fine-grained (BBC)	Improves strength and ductility, refines grain structure
Solution annealing	Austenitic stainless steel and nickel-based  alloys	Stabilized austenitic phase (FCC)	Dissolves carbides and precipitates, enhances corrosion resistance
Precipitation hardening	Nickel-based alloys and certain stainless steels (e.g.,17-4PH)	Fine precipitates in FCC/BCT matrix	Strengthens by obstructing dislocation movement, improving mechanical properties
Stabilization heat treatment	Austenitic stainless steels (e.g., 321, 347)	Austenitic phase (FCC)	Prevents chromium carbide precipitation, preserves corrosion resistance

Table 1: Examples of heat treatment and the effects on materials

Heat treatments and crystal structures define material performance. Martensitic steels, formed through rapid cooling, develop a body-centered tetragonal (BCT) structure and offer high hardness and wear resistance, ideal for valve trim and seats. Austenitic steels, with a face-centered cubic (FCC) structure, provide superior ductility, toughness and corrosion resistance, making them suitable for valve bodies and chemical processing. Ferritic steels, featuring a body-centered cubic (BCC) structure, deliver moderate strength and oxidation resistance for cost-effective industrial valves (Figure 2). (WAITING ON PERMISSION FOR THIS IMAGE)

Duplex stainless steels combine dual FCC and BCC phases (Figure 3) for high strength and corrosion resistance, requiring careful heat treatment to prevent sigma phase formation.

Nickel-based alloys, such as Inconel and Hastelloy, leverage their FCC structure to maintain oxidation and corrosion resistance under extreme conditions, making them essential for high-temperature, high-pressure valves. Cobalt-based alloys like Stellite provide outstanding wear and galling resistance, commonly used for hardfacing valve components. Titanium alloys, with a hexagonal close-packed (HCP) structure, offer an excellent strength-to-weight ratio and corrosion resistance, valuable in seawater and aerospace applications. 

Understanding heat treatments and crystal structures allows engineers to optimize valve performance and ensure reliability in demanding applications.

Metallurgy matters

Selecting the appropriate materials for valves is a complex process requiring careful consideration, not only of the material group, but also specific grades, heat treatment conditions and compliance with industry standards like NACE (National Association of Corrosion Engineers) for sour service applications. In this application, compliance ensures resistance to hydrogen sulfide (H₂S)-induced failures such as sulfide stress cracking (SSC), stress corrosion cracking (SCC) and hydrogen embrittlement, which are common failure mechanisms in oil and gas environments. To mitigate these risks, NACE imposes strict hardness limits to balance toughness and mechanical strength, enhancing durability. 

The table below provides a comparison of commonly used valve materials, highlighting their key benefits, limitations and typical applications.

Material	Key Benefits	Limitations	Typical Applications
17-4 PH	High strength, corrosion resistance	Susceptible to SCC in H900 conditions	Valve stems, trim components
Inconel 718	High strength, oxidation resistance	Lower mechanical properties above 1200°F (650° C)	High-temperature valves
Inconel 625	Superior corrosion and oxidation resistance	Lower strength  compared to Inconel 718	Higher temperature applications, oxygen service
Carbon steel (WCB)	Good mechanical properties, easy to manufacture	Brittle at low temperatures; suitable up to 800°F (425°C)	General-purpose valve bodies
LCC/LCB	Enhanced toughness for cryogenic use	Not suitable for high temperatures	LNG, cryogenic processing
321/347 SS	Stabilized for high temperatures, resists sensitization	Carbide precipitation risk at improper heat treatment	Refinery, high-temperature applications
Duplex SS (2205, 2507)	High strength, corrosion resistance	Formation of sigma phase if overheated	Offshore, chemical processing
Grade 91 (9Cr-1Mo steel)	Superior creep strength, long-term stability	Cast version has lower creep  resistance	Power plants, high-temperature refining
Titanium (grades 1-4, Ti-6AI-4V, Grade 12)	Exceptional corrosion resistance, lightweight	Lower strength in pure  form	Seawater, aerospace, mining
Monel (400, K-500)	Outstanding seawater and acid resistance	Higher cost than stainless steel	Marine, desalination, chemical plants, oxygen service
Stellite 6	Superior wear resistance, withstands high temperatures	High hardness reduces ductility	Valve seats,  high-wear parts
Stellite 21	Improved toughness, corrosion resistance	Lower hardness than Stellite 6	Stems, guide surfaces

Table 2: A comparison of commonly used materials.

For example, 17-4 PH stainless steel in the H900 condition offers high strength but is prone to SCC, making it unsuitable for NACE applications. In contrast, the H1150D condition improves toughness and corrosion resistance at the cost of some strength, making it a better choice for impact-resistant applications. Choosing the wrong heat treatment condition for 17-4 PH can lead to failure (Figure 4). 

 
Additionally, 17-4 PH is limited to operating temperatures below 480°F (250°C) as its copper-rich strengthening precipitates coarsen (over time, larger precipitates grow at the expense of smaller ones in a material) and dissolve at higher temperature, reducing mechanical properties. For higher temperature applications, Inconel 718 or Inconel 625 are preferred. Similarly, Inconel 718 undergoes aging treatments to optimize strength but loses mechanical integrity above 1202°F (650°C) due to detrimental phase formation. Inconel 625, more stable at higher temperatures, is available in Class 1 (annealed, high corrosion resistance) and Class 2 (precipitation-hardened, higher strength) is preferred.  

Carbon steels such as WCB, LCB and LCC are widely used in valve bodies, but performance varies with temperature. Standard WCB becomes brittle in cryogenic conditions, requiring low-carbon variants like LCB and LCC for improved impact resistance. Conversely, for high-temperature applications, WC6 and WC9 provide superior oxidation resistance and creep strength.
Stabilized stainless steels, such as 321 and 347, are designed for high-temperature applications where resistance to sensitization is critical to prevent chromium carbide precipitation at high temperature using titanium (321) or niobium (347) stabilizers. While thermal stabilization treatments are not always required, they are recommended for prolonged high-temperature applications like ebullated bed process valves.

Duplex stainless steels (2205, superduplex 2507) offer high strength and corrosion resistance but require precise heat treatment to prevent intermetallic phase formation, such as sigma phase, which reduces toughness and corrosion resistance (Figure 5). 

Grade 91 (modified 9Cr-1Mo), a creep-strength-enhanced ferritic (CSEF) steel widely used in power plants and refineries has two types: Type 1 (normalized and tempered) and Type 2 (with stricter tramp element control such as tin and antimony, for better long-term creep performance). Cast Grade 91 lacks these classifications and has lower creep resistance than wrought versions.

Commercially pure titanium (Grades 1–4) excels in seawater and chemical processing but lacks strength for high-pressure use. Ti-6Al-4V offers higher strength, while Grade 12 (Ti-0.3Mo-0.8Ni) resists crevice corrosion in chemical processing and mining applications.

Like titanium, Monel alloys, primarily nickel-copper, are also extensively used in marine environments due to their outstanding resistance to seawater corrosion and hydrofluoric acid. Monel 400 is common in offshore and marine valves, while K-500, precipitation-hardened for higher strength, is ideal for marine and oil and gas high-performance valves. A key difference between titanium and Monel is that titanium is much lighter, making it advantageous for weight-sensitive applications such as naval components. However, Monel has a lower risk of galling, making it a better choice for sliding components. Proper design prevents galvanic corrosion when pairing these materials. 

Stellite 6 and Stellite 21 are cobalt-based alloys used for wear and corrosion resistance. Stellite 6, with tungsten, excels in galling resistance but is harder and prone to cracking under thermal cycling (Figure 6). It performs well up to 1112°F (600°C), becoming brittle at higher temperatures. Stellite 21, with lower carbon and added molybdenum, offers improved toughness and corrosion resistance. Stellite 6 is preferred for high-wear components like valve seats, while Stellite 21 is good for stems and guide surfaces requiring improved galling resistance and thermal stability.

 

 

 

 

 

 

 

 

 

 

Challenges in material compatibility

In addition to selecting individual materials, understanding how their combination influences performance is equally important. When different metals are combined through welding, cladding or bimetallic assemblies, their interactions can impact mechanical integrity and corrosion resistance, leading to premature failure. Key considerations include:

• Thermal expansion mismatches: In valve components, differential thermal expansion between materials can cause significant issues. This mismatch can result in stress, leakage and sealing failure (Figure 7). Specifically, when base materials and coatings are used together, the differences in thermal expansion may lead to cracking, disbonding or leakage, particularly in high-temperature applications. These stresses can compromise the integrity of the valve, reducing its reliability and lifespan. In the photo below, deformation of the piping support was caused by improperly designed piping to accommodate thermal expansion under large cyclic loads. This led to excessive stresses within the piping system, contributing to valve leakage at the body/body end flange in ebullated bed unit.
  

  Figure 7. Source: Velan Inc.

• Galvanic coupling: Combining dissimilar metals having different electrochemical potentials, such as stainless steel and carbon steel, in corrosive environments can accelerate galvanic corrosion and cause rapid material degradation.
• Phase transformations: When materials are exposed to specific operating conditions over time, such as elevated temperatures or mechanical stress, phase transformations can occur. These changes may significantly alter mechanical properties like hardness, strength and ductility. In valve applications, exceeding the material’s initial metallurgical limits — especially under prolonged high-temperature or high-pressure service — can trigger these transformations. If not properly accounted for, they can cause brittleness, fatigue or reduced corrosion resistance, ultimately compromising valve integrity. For example, hardfacing CSEF steels with cobalt-chrome-based alloys can result in delamination due to the formation of brittle intermetallic phases at operating temperatures above 1000ºF (538°C) (Figure 8).
  

  Figure 8A: Hardfacing delamination. Source: Velan Inc.

   

 

 

 

 

 

 

 

 

 

Figure 8B: Microscopy of failed disc shown in image (above, 9A) reveals an intermixed zone at the interface between Grade 91 and Stellite 21, indicating the formation of hard and brittle sigma phase. Source: Velan Inc.

The future of metallurgy

The future of metallurgy in valve applications will be shaped by advanced materials, sustainability and regulatory challenges. Functionally graded materials (FGM), which gradually transition in composition and microstructure, offer enhanced thermal resistance and mechanical performance. Additive manufacturing (3D printing) allows for complex geometries and tailored material properties, enabling next-generation valve solutions.

Despite these innovations, regulatory requirements can slow adoption. Certifications from ASME, API and ASTM ensure safety but also create barriers to new material integration. To overcome this, industries must work closely with regulatory bodies, demonstrating long-term benefits through rigorous testing.

Sustainability is becoming a priority, with an increasing focus on environmentally friendly alloys, energy-efficient production techniques and the recycling of high-performance metals. Advanced coatings that extend component lifespan further contribute to sustainability efforts.
By balancing innovation with regulatory compliance, metallurgy will continue to evolve, driving improvements in valve technology while ensuring safety, performance and sustainability for future applications.

Fadila Khelfaoui is a corporate engineer, metallurgy at Velan, with 27 years of experience in materials engineering and metallurgy. She has led numerous R&D initiatives aimed at optimizing material selection and developing protective coating technologies for valve components operating in demanding industrial environments. Fadila holds a Ph.D. in Materials Science and Engineering and is a licensed metallurgical engineer in Quebec.