Post-Weld Heat Treatment Effects on Duplex Steel SMAW Overlay Microstructure and Properties

Literature Overview and Context

Duplex stainless steels (DSS), characterized by a balanced microstructure of approximately 50% austenite and 50% ferrite, offer excellent combinations of strength, toughness, and corrosion resistance. However, welding duplex steels presents unique challenges: the weld metal and heat-affected zone are susceptible to sigma phase precipitation, chromium nitride (CrN) precipitation, and loss of phase balance if not properly controlled. The 2013 study by Zhang Bin, Zhang Kai, Ren Shihong, Chen Hongwei, and Zhang Jianxiao from Lanzhou Lanchi Heavy Equipment Co., Ltd. investigates the effects of post-weld heat treatment (PWHT) on the microstructure and properties of duplex steel overlay deposits produced by shielded metal arc welding (SMAW). This is particularly relevant for pressure vessel fabrication, where duplex steel overlays are used to provide corrosion resistance in aggressive environments.

Core Technical Points

Duplex Steel Welding Challenges

Challenge	Mechanism	Consequence
Sigma phase precipitation	Long-term exposure at 600–900 °C	Loss of toughness and corrosion resistance
CrN precipitation	Rapid cooling through 1000–1200 °C	Localized depletion of Cr, pitting corrosion
Phase imbalance	Preferential solidification of one phase	Reduced strength or corrosion resistance
Intergranular corrosion	Cr depletion at grain boundaries	Reduced resistance to pitting and crevice corrosion
Hydrogen-induced cracking	Hydrogen diffusion into ferrite	Delayed cracking

Post-Weld Heat Treatment Objectives

PWHT for duplex steel welds serves multiple purposes:

Phase balance restoration: Solution treatment at 1050–1150 °C dissolves sigma phase and CrN, allowing re-equilibration of the austenite-ferrite balance
Residual stress relief: Stress relief at 550–650 °C reduces welding-induced residual stresses
Microstructural homogenization: Uniform distribution of alloying elements throughout the weld metal
Toughness improvement: Dissolution of brittle phases and refinement of microstructure

Heat Treatment Parameter Optimization

Parameter	Solution Treatment	Stress Relief	Combined Treatment
Temperature	1050–1150 °C	550–650 °C	Stress relief followed by solution
Holding time	30–60 min	1–2 hours per 25 mm thickness	Per respective requirements
Cooling method	Rapid air cooling or water quench	Furnace cool to 300 °C, then air cool	Per solution treatment
Preheat	Not typically required	100–200 °C	Per stress relief

Microstructural Evolution with PWHT

The study examines how different PWHT conditions affect the microstructure of SMAW duplex steel overlay deposits:

Without PWHT:

Phase imbalance: Often ferrite-rich in the weld metal due to solidification behavior
CrN precipitation: Present as fine particles at ferrite-austenite boundaries
Sigma phase: May be present in the HAZ if peak temperatures exceed 600 °C
High residual stress: Tensile stresses up to yield strength in the weld and HAZ

After Solution Treatment:

Phase balance: Restored to approximately 50:50 austenite:ferrite
CrN dissolution: Precipitates dissolve, allowing Cr to redistribute
Sigma phase dissolution: Brittle sigma phase dissolves, improving toughness
Reduced residual stress: Partial relief due to stress relief during cooling

After Stress Relief:

Phase balance: Unchanged (no dissolution of precipitates)
CrN: Still present
Sigma phase: Still present
Residual stress: Reduced by 50–70%

Mechanical Property Effects

Property	As-Welded	After Stress Relief	After Solution Treatment
Tensile strength (MPa)	600–700	580–680	600–700
Yield strength (MPa)	450–550	420–520	450–550
Elongation (%)	25–35	28–38	30–40
Impact energy (J)	50–100	60–120	80–150
Hardness (HV)	250–300	230–280	250–300
Pitting resistance (PREN)	32–35	30–33	33–36

Engineering Practice Integration

In pressure vessel fabrication, the selection of PWHT conditions must be guided by applicable codes and standards:

Standard	PWHT Requirement	Typical Condition
ASME VIII Div.1	Solution treatment recommended for DSS	1050–1150 °C, air cool
ASME VIII Div.2	Solution treatment required for DSS welds	Per material specification
GB/T 150	Stress relief or solution treatment per design	Per design specification
NB/T 47002	Solution treatment for DSS	1050–1150 °C
EN 13445	Solution treatment recommended	Per material specification

Practical Considerations for Pressure Vessels

Distortion control: Solution treatment at high temperatures can cause significant distortion in thick-walled pressure vessels. Controlled cooling rates and support fixtures are essential.
Stress relief compatibility: If stress relief is required in addition to solution treatment, the sequence must be carefully planned. Stress relief followed by solution treatment is generally preferred.
Post-PWHT inspection: Visual inspection, MT, and UT should be performed after PWHT to detect any cracking or other damage.
Corrosion testing: Post-PWHT corrosion testing (e.g., ASTM A262 Practice E or Practice A) should be performed to verify pitting resistance.
Hydrostatic testing: Final hydrostatic testing per code requirements must be performed after all heat treatment is complete.

Key Reflections and Study Insights

This study underscores the critical importance of PWHT in duplex steel welding applications, particularly for pressure vessel fabrication where service reliability is paramount. The findings demonstrate that while stress relief provides beneficial residual stress reduction, it does not address the fundamental metallurgical issues of phase imbalance and precipitate formation. Solution treatment, despite its higher cost and greater distortion risk, is essential for restoring the duplex microstructure and ensuring long-term corrosion resistance. A key insight from this literature is that the choice of PWHT condition must be driven by the specific service environment: for aggressive corrosive environments, solution treatment is non-negotiable; for less demanding environments, stress relief may be acceptable if the as-welded microstructure is acceptable. The SMAW process used in this study is particularly challenging for duplex steel welding due to the difficulty of controlling heat input and dilution. The use of appropriate filler metals (e.g., 2205 or 2507 grade electrodes) and careful process control are essential to produce welds that can benefit from PWHT rather than requiring excessive correction. In my experience, the most common oversight in duplex steel pressure vessel fabrication is the failure to perform post-PWHT corrosion testing, which can miss critical issues such as sensitization or inadequate solution treatment.