QUICK DEFINITION: HAZ SOFTENING IN X65/X70 LSAW PIPE WELDINGHAZ Softening is a localized reduction in strength caused by the reversion of non-equilibrium acicular ferrite to stable polygonal ferrite during welding. Governed by DNV-OS-F101 and API 5L, it is critical in offshore LSAW pipelines where strain-based design is utilized. Failures typically manifest when high heat input (>3.0 kJ/mm) extends $t_{8/5}$ cooling times, causing cross-weld tensile specimens to fracture in the HAZ below the Specified Minimum Tensile Strength (SMTS).
In the metallurgy of high-strength low-alloy (HSLA) line pipe, Thermo-Mechanical Control Process (TMCP)steels present a specific paradox. While they allow for high strength (X65, X70) with lean chemistry and excellent weldability, they are thermodynamically unstable. The manufacturing process freezes strength into the steel; the welding process releases it.
For Welding Engineers and Metallurgists dealing with LSAW (Longitudinal Submerged Arc Welding) pipe, the "Soft Zone" in the Heat Affected Zone (HAZ) is a frequent cause of procedure qualification failure. Unlike conventional steels that harden and crack, TMCP steels soften and yield. This article details the mechanism of this softening, the critical cooling time parameters, and how to navigate compliance under DNV-OS-F101.
The Metallurgical Mechanism of Softening
Why does TMCP steel lose strength when heated?
TMCP steel derives its mechanical properties from grain refinement and dislocation density achieved through controlled rolling and accelerated cooling, rather than heavy alloying. This produces a microstructure of fine-grained acicular ferrite or bainite. This state is a "non-equilibrium" condition.
During LSAW welding, the Intercritical HAZ (ICHAZ) and Fine-Grained HAZ (FGHAZ) are heated to temperatures between $A_{c1}$ (approx. 720°C) and $A_{c3}$ (approx. 850°C). This heat input acts as a catalyst, transforming the metastable acicular ferrite back into austenite. Upon cooling, if the rate is slower than the original mill cooling (which is almost guaranteed in SAW), the austenite transforms into thermodynamically stable, but mechanically weaker, polygonal ferrite and granular bainite.
Technical Clarifier: The Role of Carbon Equivalent ($P_{cm}$)
Ironically, cleaner steels can be more problematic. Ultra-low carbon X65 ($C < 0.05\%$) relies heavily on cooling rate for strength. With lower hardenability, these lean chemistries are more prone to forming soft polygonal ferrite in the HAZ if the cooling rate is not strictly controlled.
The $t_{8/5}$ Cooling Time Trap
How does cooling time dictate the severity of the Soft Zone?
The extent of softening is directly proportional to the time the weldment spends cooling from 800°C to 500°C, denoted as $t_{8/5}$.
Target Window: For X65/X70, optimal properties usually require a $t_{8/5}$ between 8 and 20 seconds.
The LSAW Reality: LSAW is a high-deposition process. Heat inputs often range from 2.5 to 4.5 kJ/mm. In heavy wall pipe (>25mm), a 3.5 kJ/mm heat input can result in a $t_{8/5}$ exceeding 30 seconds.
The Consequence: At $t_{8/5} > 25s$, the formation of blocky proeutectoid ferrite dominates the microstructure. This phase lacks the dislocation density of the base metal, leading to hardness drops of 30–60 HV10.
Negative Constraint: Do Not Apply Standard PWHT
Post-Weld Heat Treatment (PWHT) at 600°C+ is generally prohibited for TMCP X65/X70 materials. Unlike normalized steels, TMCP steels will suffer global degradation of yield strength (often dropping 50–80 MPa) if subjected to stress relief temperatures, effectively downgrading an X65 pipe to X52.
Common Field Questions About HAZ Softening in X65/X70 LSAW Pipe Welding
Why is DNV-OS-F101 strict on cross-weld tensile reduction?
DNV-OS-F101 (and ISO 3183) acknowledges the existence of the soft zone but limits its impact. The code typically permits a cross-weld tensile strength to be lower than the actual base metal strength, provided it meets the SMTS (Specified Minimum Tensile Strength). Some appendices allow for values at 95% of SMTS if Strain Based Design (SBD) is not utilized. The concern is that a wide, severe soft zone acts as a strain concentrator, leading to fracture path deviation and reduced plastic collapse capacity.
Can overmatching weld metal compensate for HAZ softening?
Yes. This is the primary mitigation strategy. By ensuring the Weld Metal (WM) yield strength significantly exceeds the Base Metal (BM) yield strength (Overmatch > 100 MPa), the stiffer weld metal creates a constraint effect. This "shielding" prevents strain localization within the narrow soft HAZ, forcing plastic deformation into the base metal during global loading events.
Does the pipe wall thickness affect the soft zone width?
Indirectly, yes. Thicker wall pipe (e.g., >30mm) acts as a more efficient heat sink, potentially lowering $t_{8/5}$ (3D heat flow). However, LSAW welding on thick wall pipe often requires multi-wire tandem SAW with massive heat input to ensure penetration, which counteracts the cooling benefit. The cumulative thermal cycles in the root and hot pass of thick-wall welds often generate the widest soft zones.
Engineering Solutions for HAZ Softening in X65/X70 LSAW Pipe Welding
Mitigating HAZ softening requires a combination of precise material selection and controlled welding parameters. When procuring pipe, ensuring the chemical composition has sufficient hardenability (via Mn, Mo, or Ni additions) to resist ferrite formation at slower cooling rates is essential.
Furthermore, selecting the correct pipe manufacturing method is the first line of defense. For large diameter high-pressure lines, LSAW produced with strict TMCP protocols is required to maintain toughness while minimizing the soft zone width.
Recommended Product Integration:
For Large Diameter High-Pressure Transmission: Utilize high-grade Welded Line Pipe (LSAW) engineered with specific chemistry for offshore and sour service applications.
For High-Pressure Flowlines (Smaller Diameter): Consider Seamless Line Pipe where the Quench & Temper (Q&T) process provides a more uniform microstructure less susceptible to the same softening mechanisms as TMCP.
FAQ: Tribal Knowledge on TMCP Softening
What is the maximum acceptable hardness drop in X65 HAZ?
While codes like DNV-OS-F101 do not set a strict "minimum hardness" for rejection, a drop of more than 40-50 HV10 below the base metal average is a significant warning sign. It indicates a microstructure capable of strain localization. Most operators aim to keep HAZ hardness above 180-190 HV10 for X65 grades.
How do you calculate the $t_{8/5}$ cooling time for LSAW?
For thick plates (3D heat flow), a field rule of thumb is $t_{8/5} \approx (6700 \times E) - 5$, where E is heat input in kJ/mm. However, strict numerical modeling or direct thermocouple measurement during Procedure Qualification Records (PQR) is required for accuracy, as preheat and interpass temperature significantly skew this value.
Why does the root pass suffer the most softening?
The root pass (and adjacent HAZ) is subjected to multiple reheating cycles from subsequent fill passes. These thermal cycles can temper the already softened structure or repeatedly cycle it through the intercritical range, promoting grain coarsening and further hardness reduction.
Can normalizing the pipe after welding fix the soft zone?
Full Normalizing (heating above $A_{c3}$ and air cooling) will eliminate the soft zone but will typically destroy the mechanical properties of the TMCP base metal. TMCP achieves X65/X70 strength via rolling practice; normalizing resets the grain structure, likely dropping the strength to Grade B or X42 levels unless the steel has heavy alloying (which TMCP usually does not).
