Duplex stainless steel is a high-performance alloy known for its impressive combination of high strength and exceptional corrosion resistance. In demanding industries like offshore oil and gas, chemical processing, and marine engineering, fasteners made from this material are indispensable. They are the critical components holding together systems that face saltwater, high pressures, and corrosive chemicals.
But these remarkable properties are not inherent in the raw material alone. They are the end product of a highly controlled and complex manufacturing process. Unlike a standard steel bolt, a duplex bolt’s performance is entirely dependent on achieving a precise, two-phase microscopic structure.
If this process is handled incorrectly, the bolt can look perfect on the outside but be an operational failure waiting to happen—brittle, weak, and with compromised corrosion resistance. This article explains the step-by-step manufacturing journey of a duplex stainless steel bolt, from raw bar stock to a finished, certified component.
The Goal: A Perfect 50/50 Microstructure
The name “duplex” refers to the steel’s microstructure, which is a “duplex” or two-phase structure containing a balanced mix of roughly 50% austenite and 50% ferrite.
- Austenite provides the excellent corrosion resistance and weldability.
- Ferrite provides the high strength and resistance to stress corrosion cracking.
The entire manufacturing process, especially the heating and cooling stages, is a delicate balancing act. The goal is to encourage this 50/50 split and, just as importantly, prevent the formation of harmful third phases that can destroy the bolt’s properties.
The Manufacturing Journey: A Step-by-Step Guide
Creating a duplex bolt is a process of metallurgical science. It involves far more than just cutting and shaping metal.
Step 1: Raw Material Sourcing and Verification
The process begins with the raw material: high-quality duplex stainless steel bar stock in a grade like S31803 or S32205 (also known as 2205). Reputable manufacturers source this bar stock from certified mills.
Upon arrival, the material is verified. This often includes:
- Reviewing the Material Test Certificate (MTC): This document from the mill confirms the exact chemical composition (the percentages of chromium, nickel, molybdenum, nitrogen, etc.) and mechanical properties.
- Positive Material Identification (PMI): A handheld XRF analyzer is often used to quickly confirm the alloy grade, ensuring the bar is indeed the duplex grade it claims to be.
Step 2: Cutting and Forming (Forging)
Once the material is approved, the bar stock is cut into “blanks” of the correct length. These blanks are then formed into the basic shape of the bolt, including the head. This is typically done in one of two ways.
- Cold Forging (Cold Heading): For smaller-diameter bolts and high-volume production, the blanks are fed into a machine that shapes them at room temperature using a series of high-pressure dies. This process work-hardens the steel, increasing its strength.
- Hot Forging: For larger-diameter bolts, custom heads, or complex shapes, the blanks must be heated to make them malleable. They are heated to a specific temperature (e.g., 1100°C to 1250°C) and then pressed into shape by a die.
This heating step, while necessary for forging, disrupts the steel’s carefully balanced microstructure. This makes the next step the most critical part of the entire process.
Step 3: The Critical Phase—Solution Annealing (Heat Treatment)
Whether the bolt was hot-forged or cold-forged, it must undergo a specific heat treatment called “solution annealing” or “solution treatment.” This process is designed to do two things:
- “Reset” the Microstructure: It re-dissolves any harmful intermetallic phases that may have started to form.
- Achieve the 50/50 Balance: It ensures the proper austenite-ferrite phase balance is restored throughout the bolt.
The process involves heating the bolts to a precise temperature, typically between 1020°C and 1100°C, and holding them there for a specific, calculated time.
Step 4: The Rapid Quench
What happens immediately after solution annealing is just as important as the heating itself. The bolts must be cooled extremely quickly, almost instantaneously. This is almost always done by dropping them from the furnace directly into a large water bath.
This rapid “quench” is essential. The “danger zone” for duplex steel is between approximately 950°C and 650°C. If the steel is allowed to cool slowly through this window, a brittle, unwanted “sigma phase” will form, which is the primary enemy of duplex steel.
The Enemy: Understanding Sigma Phase
The “sigma phase” is a hard, brittle, intermetallic phase rich in chromium and molybdenum. It is the single biggest threat to a duplex bolt’s integrity.
- How it Forms: It forms when the steel is held at or cooled slowly through its “danger zone” (650-950°C). This can happen during improper hot forging, incorrect annealing, or even during welding if procedures are not followed.
- What it Does:
- Embrittlement: It makes the steel incredibly brittle, destroying its toughness and impact strength.
- Corrosion: Because it “eats” the chromium and molybdenum from the surrounding metal, it starves the steel of the very elements that give it corrosion resistance.
A bolt with sigma phase might pass a simple dimensional check, but it will fail catastrophically in service, either by cracking under load or corroding rapidly. The rapid water quench after solution annealing is the only way to “lock in” the correct, tough, and corrosion-resistant structure.
Step 5: Threading
After the bolts have been heat-treated and quenched, they are ready for threading. While threads can be cut, the superior method is thread rolling.
In this cold-forming process, the bolt blank (which is now properly heat-treated) is rolled between two hard dies that press the thread shape into the metal. This is preferable because:
- It creates a stronger thread by work-hardening the root.
- It improves fatigue resistance.
- It results in a smoother surface finish, which reduces the risk of “galling” (the friction-based seizing common in stainless steel fasteners).
Step 6: Finishing, Passivation, and Final QC
The bolts are nearly complete. The final steps involve cleaning and surface treatment.
- Descaling: Any oxides or “scale” from the heat treatment are removed, often through pickling (a chemical bath).
- Passivation: The bolts are placed in a passivating bath (typically nitric or citric acid). This process removes any free iron from the surface and helps to build up the all-important passive chromium-oxide layer, which is what gives stainless steel its corrosion protection.
Finally, the finished bolts undergo a rigorous Quality Control (QC) inspection. This is what separates a world-class supplier from a simple job shop. A qualified duplex stainless steel bolts manufacturer will conduct a range of tests, which may include:
- Dimensional Checks: Verifying the thread, head, and length.
- Mechanical Testing: Pulling sample bolts to test their ultimate tensile strength and yield strength.
- Impact Testing (Charpy Test): Striking a notched sample with a pendulum to ensure it meets toughness requirements and has no embrittlement.
- Microstructure Analysis: Cutting a bolt, polishing it, and examining it under a microscope to visually confirm the 50/50 phase balance and, most importantly, the absence of the dreaded sigma phase.
This rigorous, science-based approach ensures that every bolt that leaves the facility will perform as expected in the most critical applications.
For a reliable supply of duplex stainless steel fasteners that have been correctly manufactured, heat-treated, and certified, you can contact Amco Metals.