Is PWHT Required For Stainless Steel Weldments?

Though progress in high temperature applications of stainless steel has been significant, significant issues still persist with weldments due to reheat cracking issues.

Reheat cracking typically begins due to sensitization in the base metal heat affected zone (HAZ) and weld, leading to precipitation of chromium carbides.

Carbon Content

Carbon content of steel determines its strength, ductility and corrosion resistance as well as welding and post weld heat treatment (PWHT) requirements. In stainless steels it can range from 0.03 to 0.5% to increase hardness and wear resistance; however excessive amounts can make the steel brittle and less weldable.

Stainless steel grades often incorporate low-carbon alloying elements, like chromium, molybdenum and manganese, into their makeup in order to improve properties. When combined, these elements reduce carbon content to below 0.25 percent allowing for weldability, increased corrosion resistance and lower machinability compared to carbon steels.

When it comes to PWHT, stainless steel is usually the superior choice over carbon steel. Its lower carbon content makes welding and handling simpler across all manufacturing processes – something which is particularly crucial during machining operations. Furthermore, having lower yield stress makes stainless steel less vulnerable to denting or bending compared to carbon steels.

Austenitic stainless steels tend to feature low carbon contents of up to 0.10% and can often be weldable without postweld heat treatment (PWHT). When joined to carbon steels however, these grades become more susceptible to intergranular corrosion caused by differential thermal expansion between both types of metal. Therefore, knowing your stainless steel grade’s carbon content before choosing PWHT treatment is of great significance.

Overall Chemistry

The alloys that comprise stainless steel can vary considerably in composition. Different ratios of iron, chromium, nickel and molybdenum contribute to its composition; their exact mixtures depending on what type of stainless steel alloy is needed and its intended use. These factors influence factors like strength, corrosion resistance, weldability, magnetism density and thermal conductivity among other characteristics.

As an example, certain grades of stainless steel can withstand acidic solutions, while others cannot. By increasing chromium and molybdenum content in stainless steel alloys, resistance against reducing acids like hydrochloric acid, dilute sulfuric acid and nitric acid increases. Unfortunately, however, this will not increase resistance against oxidizing acids like concentrated sulfuric acid.

Stress corrosion cracking (SCC), another hallmark of stainless steel grades, occurs when exposed to chlorides for extended periods and at elevated temperatures, and results in cracks developing within its metal. If left uncorrected, SCC can lead to metal failure resulting in failure or cracks forming that are ultimately fatal.

Stainless steels offer excellent durability and can withstand various environments, which makes them highly desirable materials in manufacturing. Their resilience means they can withstand chemical plants, oil & gas pipelines and more while remaining fireproof – another reason they’re used extensively in construction and car exhaust systems.

Welding Temperature

Stainless steel is highly corrosion-resistant, making it an excellent choice for restaurant kitchens, craft breweries, medical equipment and transportation of liquids and gases. Stainless steel can withstand extreme hot and cold temperatures as well as many forms of chemical and mechanical stress; additionally it’s very durable, being able to withstand abrasions and impacts without cracking.

Depending on the composition of your stainless steel, additional precautions may be necessary when welding it. Fully austenitic grades (300 series) can become crack prone if left at elevated temperatures for extended periods, since heat input can alter the balance between ferrite and austenite in both weld metal and base metal, leading to eventual Sigma phase formation and cracking.

Ferritic and duplex stainless steels require extra care due to their lower maximum interpass temperature limits and require specific preheating and postweld heat treatment temperatures in order to prevent unwanted phases from forming during welding.

An effective PWHT can reduce SMSS-IGSCC sensitivities due to carbide precipitation in chromium-depleted zones near weld interfaces, and reduce hydrogen-related defects in joints while increasing strength, ductility, corrosion resistance and pitting resistance in mildly acidic environments.

Section Thickness

Stainless steel can be found in numerous applications and is often chosen as a material for structures due to its strength and resistance to corrosion. However, its strength is determined not only by the grade and alloy composition of its sheet but also its thickness; standard gauge thickness affects how easily bending or warping occurs, with thicker sheets more resistant but requiring special tools for fabrication.

Measured using a gauge chart, the thickness of stainless steel sheets can be determined using their gauge number. Although its gauge number doesn’t provide much insight into their actual thickness, different metals have different gauge charts and can differ considerably in thickness from one another. When selecting materials for projects of specific dimensions it is crucial that Gauge charts be checked to confirm availability in desired sizes.

Post weld heat treatment (PWHT) of stainless steel depends on its grade and application. Austenitic stainless steels require minimal PWHT due to their low carbon content and outstanding weldability; martensitic and precipitation hardening grades may benefit from PWHT treatments; during which, the weldment should be gradually heated to its targeted temperature before remaining at that level for a specified duration of time.