Duplex and Super Duplex. Ensuring Your Fastener is Correct

When purchasing anything, you always want to ensure you get exactly what you ordered. With fasteners, this is no different and is becoming more and more critical in the high end markets of Oil & Gas,Petrochemical and Offshore bolting.

In the past 5 years, the use of Duplex and Super Duplex has increased dramatically in the fastener industry. Raw materials are readily available, in a good range of sizes and competitively priced. These factors have encouraged anumber of companies to get involved with these materials, meaning competitionis fierce. However, Duplex materials are complicated and, if not handled rightat every stage, can give end users major problems.

The main reasons these problems occur is when a product, such as a Hexagon Bolt, Socket Capscrews or Hexagon nut is produced by hot forging. The hotforging process is either performed incorrectly, the heat treatment load is not done and tested to a repeatable and approved procedure, or sometimes productsare not even heat treated at all.

Imagine this scenario; You have 2 hexagon bolts that have been hot forged andmachined, that look exactly the same, from 2 different suppliers. All thematerial certificates conform to the required standard and a PMI check of eachfastener shows both are the correct grade. However, only one supplier hasn’tcarried out the hot forging and heat treatment process correctly. The only wayyou could now fully tell the difference is by carrying out a full metallurgical analysis on the finished products.

Duplex materials have an austenite and ferrite balance,which gives a better corrosion resistance (stress and pitting), plus a higherstrength than standard austenitic stainless steels (A2/A4). However, when hot forged or heat treated incorrectly, this balance can be altered and these increased capabilities are altered or completely taken away. There are also detrimental phases that can be formed, such as Sigma and Chi, should the material not be handled correctly. These phases can greatly reduce corrosion resistance and the strength performance expected of Duplex and Super Duplex.

After the hot forging of these stainless steel wire, there should always be a solution treatment take place.This solution treatment process is a vital part of ensuring the material is returned to the original, intended condition it was designed for. As with any process as vital as this, the load must be treated at the correct time and temperature, followed by rapid water quenching. If the products are loaded inaccurately, heated at wrong times / temperatures or quenched incorrectly, you will have defective products.

Once heat treatment has been completed, the only way that you can ensure it has been done correctly is by carrying out specific tests on the load. These tests check the structure of the material, ensuring that the balance of austenite and ferrite is back at the correct levels, plus the detrimental phases have notoccurred. These tests are carried out by fully independent, UKAS approved testhouses. Below, you will see examples of both bad and good results on a heattreatment load:

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