CLADDING TECHNOLOGY SHANXI CO., LTDER308L Wire for Surface Layer Cladding in Stainless Steel Overlay Welding
Literature Overview
ER308L is the most widely used filler metal for the surface (face) layer of stainless steel overlay cladding systems, providing the primary corrosion-resistant barrier in welded clad pressure vessels, heat exchangers, and storage tanks. Its composition mirrors that of the 304L austenitic stainless steel grade, offering excellent general corrosion resistance, good formability, and excellent weldability. This study note provides a comprehensive analysis of ER308L surface layer cladding, covering composition, process optimisation, ferrite number control, dilution management, and quality assurance practices.
Compositional Analysis and Metallurgical Properties
ER308L is a low-carbon austenitic stainless steel wire electrode with a composition designed to produce weld metal matching the 304L grade. The low carbon content (≤0.030 wt.%) prevents sensitisation and intergranular corrosion, while the balanced chromium and nickel content provides a stable austenitic microstructure with beneficial delta ferrite.
| Property | ER308L Specification | Typical Values |
|---|---|---|
| Cr (wt.%) | 19.0 - 22.0 | 19.5 - 21.0 |
| Ni (wt.%) | 9.0 - 12.0 | 10.0 - 11.5 |
| C (wt.%) | ≤ 0.030 | 0.010 - 0.025 |
| Mn (wt.%) | 0.5 - 2.0 | 1.0 - 1.5 |
| Si (wt.%) | 0.3 - 1.0 | 0.5 - 0.8 |
| Mo (wt.%) | ≤ 0.75 | 0.1 - 0.2 |
| Ferrite number (FN) | 4 - 12 | 5 - 9 |
| Yield strength (MPa) | ≥ 205 | 220 - 280 |
| Tensile strength (MPa) | ≥ 515 | 530 - 600 |
| Elongation (%) | ≥ 30 | 35 - 45 |
| Hardness (HV) | ≤ 250 | 180 - 230 |
The absence of molybdenum in ER308L distinguishes it from ER316L and limits its resistance to chloride-induced pitting and crevice corrosion. In applications where the service environment contains chlorides above 500 mg/L, ER316L is preferred for the surface layer. However, for general-purpose corrosion resistance in mild environments, ER308L provides excellent performance at a lower cost.
Ferrite Number Control and Microstructural Engineering
The ferrite number (FN) is the single most important metallurgical parameter governing the performance of ER308L surface layer welds. The delta ferrite content directly influences the weld's susceptibility to hot cracking, solidification cracking, and intergranular corrosion.
Ferrite Number and Performance Relationship
| Ferrite Number (FN) | Delta Ferrite (%) | Hot Cracking Resistance | Corrosion Resistance | Ductility |
|---|---|---|---|---|
| 0 - 4 | < 1 | Poor | Excellent | Excellent |
| 4 - 8 | 1 - 5 | Good | Very good | Very good |
| 8 - 12 | 5 - 10 | Excellent | Good | Good |
| 12 - 15 | 10 - 15 | Excellent | Fair | Fair |
| > 15 | > 15 | Excellent | Poor | Poor |
The optimal ferrite number range for ER308L surface layer cladding is 4 to 12, with a target of 6 to 9 for general applications. This range provides an optimal balance between hot cracking resistance and corrosion performance. The delta ferrite formed during solidification acts as a crack arrestor, preventing the propagation of solidification cracks that can form in fully austenitic welds due to the columnar grain structure and the Laves phase precipitation at high nickel levels.
The measurement of ferrite number is performed using a portable ferriteScope instrument per ASTM A913 or ASTM E1036. The measurement should be taken on the solidified weld bead surface after grinding to a smooth finish, and the reading should be averaged over multiple measurement points across the weld width. A ferrite number below 4 indicates a fully austenitic weld with poor hot cracking resistance, while a ferrite number above 15 indicates excessive delta ferrite with potential corrosion performance degradation.
Process Parameters and Welding Procedure Optimisation
The welding process parameters for ER308L surface layer cladding must be optimised to achieve the target ferrite number, minimise dilution from the underlying ER309L transition layer, and ensure sound weld quality.
GMAW (MIG) Surface Layer Parameters
| Parameter | Recommended Range | Notes |
|---|---|---|
| Wire diameter | 0.9 - 1.2 mm | 1.0 mm most common |
| Shielding gas | Ar 98% + CO₂ 2% | CO₂ increases ferrite |
| Gas flow rate | 12 - 18 L/min | Adequate coverage critical |
| Current (DCEN) | 180 - 280 A | Higher current = lower FN |
| Voltage | 20 - 26 V | Higher voltage = lower FN |
| Travel speed | 250 - 500 mm/min | Higher speed = lower FN |
| Stick-out | 8 - 12 mm | Longer stick-out = lower FN |
| Preheat temperature | 50 - 100 °C | Minimal preheat needed |
| Interpass temperature | ≤ 150 °C | Tighter control than transition |
TIG (GTAW) Surface Layer Parameters
| Parameter | Recommended Range | Notes |
|---|---|---|
| Electrode | ZrO₂ or LaB₆ tungsten | Non-thoriated preferred |
| Shielding gas | Pure Argon | No CO₂ for TIG |
| Gas flow rate | 8 - 12 L/min | Adequate coverage |
| Current | 120 - 220 A | DCEN |
| Voltage | 12 - 18 V | Arc stability critical |
| Travel speed | 60 - 200 mm/min | Slower for better fusion |
| Filler rod | ER308L, 2.4 mm | Continuous feeding |
| Preheat temperature | 50 - 100 °C | Minimal preheat |
The key process parameter for controlling ferrite number in GMAW welding is the current-to-travel-speed ratio. Higher current with lower travel speed increases the heat input, which promotes the dissolution of delta ferrite during solidification, resulting in lower ferrite numbers. Conversely, lower current with higher travel speed increases the cooling rate, promoting delta ferrite retention and higher ferrite numbers. The shielding gas composition also plays a role, with the addition of CO₂ increasing the ferrite number due to the stabilising effect of carbon on the ferrite phase.
Dilution Control and Layer Thickness Optimisation
The dilution from the underlying ER309L transition layer into the ER308L surface layer is a critical factor affecting the final composition and properties of the cladding. In the first pass of the surface layer, dilution from the transition layer can reach 20 to 30 percent, progressively decreasing in subsequent passes.
| Pass Number | Approximate Dilution (%) | Expected FN | Composition Shift |
|---|---|---|---|
| 1 | 20 - 30% | 8 - 12 | Slightly elevated Ni, Cr |
| 2 | 10 - 20% | 6 - 10 | Near ER308L composition |
| 3 | 5 - 10% | 5 - 8 | Full ER308L composition |
| 4+ | < 5% | 5 - 7 | Stable ER308L composition |
A minimum of three passes is recommended for the surface layer to ensure that the top two passes achieve near-full ER308L composition with minimal dilution from the transition layer. The total cladding thickness should be selected based on the erosion-corrosion allowance, with a minimum of 3.0 mm recommended for general service and 5.0 mm for severe environments.
Quality Control and Inspection Protocols
The quality assurance of ER308L surface layer cladding requires a comprehensive inspection regime covering chemical composition, microstructure, mechanical properties, and surface integrity.
| Inspection Method | Frequency | Acceptance Criteria |
|---|---|---|
| OES chemical analysis | Per batch or per heat | Within ER308L specification |
| Ferrite number measurement | Every 50 m² or per vessel | FN 4 - 12 |
| Hardness testing | Every 10 m² or per vessel | ≤ 250 HV |
| UT (phased array) | 100% of critical areas | No indications per ASME V |
| MT | 100% of surface layer | No indications per ASME V |
| PT | 100% of surface layer | No indications per ASME V |
| Intergranular corrosion test | Per batch | Pass per ASTM A923 Practice A |
| Peel/bond strength test | Per vessel or per lot | ≥ 20 MPa |
The intergranular corrosion test per ASTM A923 Practice A is particularly important for ER308L surface layers, as it verifies that the low carbon content is effective in preventing sensitisation. The test involves immersion in a 63% H₂SO₄/H₂O₂ solution at 60 °C for 24 hours, with pass/fail criteria based on weight loss and visual examination.
Common Defects and Countermeasures
Several common defects are encountered in ER308L surface layer cladding, each requiring specific countermeasures.
| Defect | Cause | Countermeasure |
|---|---|---|
| Hot cracking | Low FN (< 4), high S/P content | Increase FN to 6-9, use low-S filler |
| Lack of fusion | Low current, high travel speed | Increase current, reduce speed |
| Porosity | Inadequate shielding, contamination | Improve gas coverage, clean substrate |
| Excessive dilution | Single-pass deposition, high current | Multi-pass, reduce current |
| Surface oxidation | Inadequate back-purging | Increase purge gas flow |
| High hardness (>300 HV) | Excessive dilution, martensite | Add transition layer, reduce dilution |
| Intergranular corrosion | Carbon pickup, sensitisation | Verify low-C composition, IG test |
Study Insights and Conclusions
The study of ER308L as a surface layer filler metal reveals its role as the workhorse of stainless steel overlay cladding, providing a reliable corrosion-resistant barrier for a wide range of industrial applications. The key engineering insight is that the ferrite number is the primary control parameter governing weld quality, and its optimisation requires careful attention to process parameters, particularly the current-to-travel-speed ratio and shielding gas composition.
The dilution behaviour from the underlying ER309L transition layer is a critical consideration, and a minimum of three passes is recommended to achieve stable ER308L composition in the upper surface layers. The quality control regime should include ferrite number measurement, hardness profiling, and intergranular corrosion testing to verify the metallurgical integrity of the cladding.
In conclusion, ER308L surface layer cladding is a mature and well-understood technology that requires disciplined process control and rigorous quality assurance to achieve consistent performance. Engineers should adopt a systematic approach to weld procedure qualification, including metallographic verification, ferrite number measurement, and intergranular corrosion testing, to ensure the long-term reliability of clad components in service. The continued evolution of welding technology, including hot-wire TIG and advanced GMAW systems, offers opportunities to further improve the efficiency and quality of ER308L surface layer deposition, but the fundamental metallurgical principles governing ferrite number control and dilution management remain unchanged.