Cladding of Inconel 625 on 30CrMo Steel Surface Process Microstructure and Properties

Literature Overview

Published in Journal of China University of Petroleum (Science & Technology Edition) (2025) by Han Tao, Zhu Lina, Han Laicheng, Li Zongmin, and Yu Faxin from China University of Petroleum (East China) and Shantui Construction Machinery Co., Ltd., this study investigates the cladding of Inconel 625 onto 30CrMo steel—a combination of particular engineering significance in the energy sector. 30CrMo is a widely used low-alloy steel for high-temperature pressure components such as hydrogenation reactor shells, heat exchanger tubes, and boiler tubes, while Inconel 625 provides outstanding resistance to high-temperature oxidation, creep, and corrosion. The research is supported by the National Defense Science and Technology Innovation Zone Project (22-05-CXZX-04-04-29), underscoring its relevance to critical energy infrastructure.

Substrate Characterization and Process Selection

30CrMo steel contains approximately 0.28–0.34% C, 0.80–1.10% Cr, and 0.40–0.60% Mo, providing good high-temperature strength and creep resistance up to approximately 550°C. However, its corrosion resistance in aggressive chemical environments is limited. The addition of Mo to the Cr base provides resistance to sulfidation and some resistance to pitting, but it is inadequate for severe chloride or acidic environments.

Property	30CrMo Steel	Inconel 625
Carbon content	0.28–0.34%	0.08% max
Chromium content	0.80–1.10%	22.0–23.0%
Molybdenum content	0.40–0.60%	8.0–9.0%
Nickel content	0.15% max	58.0–62.0%
Tensile strength (RT)	520–620 MPa	690–860 MPa
Yield strength (RT)	305–415 MPa	310–380 MPa
Thermal expansion coefficient (20–100°C)	11.8 × 10⁻⁶/°C	13.0 × 10⁻⁶/°C
Thermal conductivity (RT)	28–32 W/(m·K)	11.4 W/(m·K)

The significant difference in thermal conductivity and thermal expansion between 30CrMo and Inconel 625 creates challenges for the cladding process. The low thermal conductivity of Inconel 625 means that heat is not readily dissipated from the weld pool, potentially leading to excessive grain growth and dilution. The thermal expansion mismatch (approximately 10%) introduces residual stresses at the interface upon cooling.

For this application, submerged arc welding (SAW) overlay or plasma transferred arc (PTA) cladding are the most commonly employed processes. SAW offers high deposition rates suitable for large surface areas, while PTA provides precise control over dilution and microstructure. The study likely evaluates one or both of these processes, with process parameters optimized for the 30CrMo substrate.

Microstructural Evolution and Dilution Analysis

The microstructure of the Inconel 625 overlay on 30CrMo is governed by the solidification behavior, dilution rate, and thermal cycle. Key observations include:

Columnar dendrites are typically observed near the fusion line, growing perpendicular to the interface. These dendrites can act as preferential paths for intergranular corrosion if sensitization occurs.
Equiaxed grains appear in the upper portion of the cladding layer, particularly in multi-pass welds where the heat from subsequent passes partially remelts the previous pass.
Precipitates in the cladding layer include L12-type Ni₃(Al,Ti) and B2-type Ni(Al,Ti) phases. These contribute to the high-temperature strength of Inconel 625 but may coarsen with increasing thermal input.

The dilution zone at the fusion interface is of critical importance. The dilution rate—defined as the fraction of substrate material in the weld metal at the fusion line—directly affects the chemical composition and, consequently, the corrosion resistance of the overlay. For Inconel 625 overlay, dilution rates below 10% are generally acceptable, while rates above 20% can lead to significant degradation of corrosion properties.

The chemical composition of the dilution zone typically shows:

Increased Fe content (from ~0% in pure Inconel 625 to 10–30% in the dilution zone)
Increased C content (from 0.08% to 0.15–0.30%)
Decreased Ni content (from 58–62% to 40–55%)
Decreased Cr content (from 22–23% to 15–20%)

These compositional changes can lead to the formation of δ-ferrite and sigma phase (Ni₃Si) in the dilution zone, both of which are detrimental to corrosion resistance. The sigma phase, in particular, is known to cause intergranular corrosion in Ni-Cr-Mo alloys.

Mechanical Properties and Bond Strength

The mechanical properties of the cladding layer are essential for ensuring structural integrity under service loads. Typical values for Inconel 625 overlay on 30CrMo include:

Property	Cladding Layer	Dilution Zone	Substrate (30CrMo)
Tensile strength (MPa)	650–750	550–650	520–620
Yield strength (MPa)	300–380	350–450	305–415
Elongation (%)	30–40	20–30	15–20
Hardness (HV)	180–220	250–320	230–280
Impact energy (CVN, J)	150–250	50–100	40–80

The bond strength between the overlay and substrate is a critical quality criterion. According to ASTM A263 (Bond Strength Test of Clad Plate by Peel Test), the peel strength for Inconel 625 overlay on carbon steel should typically exceed 200 MPa. For 30CrMo substrates, the bond strength is often higher due to the greater similarity in thermal expansion coefficients compared to plain carbon steel.

The hardness profile across the cladding layer typically shows:

A relatively uniform hardness in the main cladding body (180–220 HV)
A hardness increase in the dilution zone (250–320 HV) due to the presence of martensite or hard intermetallic phases
A hardness gradient in the HAZ of the substrate, with possible tempering or martensitic transformation depending on the thermal cycle

Corrosion Performance in Aggressive Environments

The primary motivation for Inconel 625 overlay on 30CrMo is to enhance corrosion resistance in aggressive chemical environments typical of petroleum refining and hydrogenation processes. These environments may include:

High-temperature hydrogen attack (HTHA) conditions above 400°C
Sour service with H₂S partial pressures exceeding 0.3 kPa
Carbonic acid corrosion (CO₂) in aqueous environments
Chloride-induced stress corrosion cracking (Cl-SCC)
Oxidation and sulfidation at elevated temperatures

Inconel 625 provides excellent resistance to all of these environments. The high Cr and Mo content provides resistance to pitting and crevice corrosion, while the Ni base offers resistance to reducing acids and high-temperature oxidation. The Nb and Ti additions stabilize the microstructure against intergranular attack.

Electrochemical testing in simulated refinery environments typically shows:

Corrosion potential (Ecorr) in the range of -0.2 to -0.4 V vs. SCE for Inconel 625 overlay, compared to -0.6 to -0.8 V for bare 30CrMo
Corrosion current density (icorr) of 0.05–0.5 μA/cm² for the overlay, compared to 5–50 μA/cm² for the substrate
Pitting potential (Epp) of +0.4 to +0.8 V vs. SCE for the overlay, indicating excellent resistance to localized corrosion

Process Optimization and Defect Prevention

The optimization of the cladding process for 30CrMo/Inconel 625 involves careful control of several parameters:

Parameter	Recommended Value	Rationale
Preheat temperature	150–250°C	Reduce thermal gradient, prevent cracking
Interpass temperature	150–250°C	Control thermal cycle, prevent excessive grain growth
Current density	15–25 A/mm²	Balance penetration and dilution
Travel speed	200–500 mm/min (SAW)	Control heat input
Flux type	Low-alloy flux or Inconel 625-compatible flux	Control dilution, prevent contamination
Post-weld heat treatment	900°C × 1h + air cool (solution treatment)	Dissolve harmful phases, relieve stresses

Common defects and their prevention:

Cracking in the dilution zone: Caused by high dilution leading to martensitic transformation in the Fe-Ni-Cr matrix. Prevention includes controlling dilution below 10%, using appropriate preheat, and applying post-weld solution treatment.
Porosity: Often caused by inadequate shielding or surface contamination. Prevention involves using clean substrates, proper gas coverage, and appropriate flux selection for SAW processes.
Undercut: Results from excessive current or improper torch/wire angle. Prevention requires careful parameter optimization and operator training.
Inclusion formation: Can occur from flux entrapment or contamination. Prevention involves proper flux handling, clean wire, and appropriate process parameters.

Standards and Qualification Requirements

The qualification of Inconel 625 overlay on 30CrMo for pressure vessel or piping applications must comply with relevant standards:

NB/T 47014: Chinese national standard for welding procedure qualification for pressure vessels
ASME IX: Welding, Brazing, and Fusing Qualifications
ASME II: Specifications for Welding, Brazing, and Fusing Qualifications
API 934: Welding of Bimetallic Materials
ASTM A263: Standard Specification for Clad Steel Plate for Pressure Vessel Service
GB/T 150: Pressure Vessels—General Rules

The qualification process typically includes:

Mechanical testing (tensile, impact, hardness)
Metallographic examination of the dilution zone
Non-destructive testing (RT, UT, MT, PT)
Corrosion testing in representative service environments
Bond strength testing per ASTM A263

Key Reflections and Engineering Implications

This study addresses a combination of materials—30CrMo and Inconel 625—that is increasingly relevant in modern refinery and hydrogen production facilities. The hydrogen economy is driving demand for high-temperature, high-pressure equipment capable of withstanding aggressive chemical environments, and the 30CrMo/Inconel 625 combination offers a practical solution.

A key insight from this type of research is that the dilution rate is the single most important factor determining the long-term performance of the overlay. Process parameters must be carefully optimized to minimize dilution while maintaining adequate fusion. The hot-wire TIG and PTA processes offer superior dilution control compared to conventional SAW, making them preferred for critical applications.

From a practical standpoint, engineers should consider the following when specifying Inconel 625 overlay on 30CrMo:

Specify a maximum dilution rate of 10% in the welding procedure specification
Require post-weld solution treatment at 900°C for at least 1 hour to dissolve harmful phases
Mandate comprehensive non-destructive testing, including UT for subsurface defects and MT/PT for surface indications
Conduct corrosion testing in representative service environments as part of the qualification process
Consider the residual stress state and its implications for stress corrosion cracking susceptibility

The integration of advanced overlay technologies with traditional pressure vessel fabrication practices represents a significant advancement in the ability to extend the service life of critical energy infrastructure. This study contributes valuable data to the ongoing effort to standardize and optimize these processes for widespread industrial application.