CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of TH-950HN Electrode Overlay Deposits
Literature Overview
This research examines the microstructural characteristics and mechanical properties of overlay deposits produced using TH-950HN coated welding electrodes on carbon steel substrates. TH-950HN is a specialized hardfacing electrode designed for applications requiring high wear resistance under abrasive and impact conditions, such as mining equipment, cement mill liners, and earthmoving machinery components. The electrode deposits a complex microstructure composed of martensite, retained austenite, and hard carbide phases. The study employs metallographic examination, X-ray diffraction (XRD) analysis, hardness mapping, and wear testing to characterize the deposit behavior under different welding parameters and heat input conditions.
Microstructural Characterization
The overlay deposits from TH-950HN electrodes exhibit a heterogeneous microstructure that evolves from the substrate interface to the weld surface. Near the fusion line, the microstructure consists primarily of tempered martensite with a hardness of approximately 380-420 HV, resulting from the interaction between the base metal and the deposit during solidification. Moving away from the fusion line toward the surface, the microstructure transitions to a mixture of fresh martensite, retained austenite, and dispersed carbides including M₇C₃, M₆C, and M₂₃C₆ types. The carbide volume fraction increases significantly in the upper layers, reaching approximately 15-25% in the top 2-3 mm of the deposit.
| Depth from Surface (mm) | Hardness (HV30) | Carbide Volume Fraction (%) | Retained Austenite (%) | Dominant Phase |
|---|---|---|---|---|
| 0.0-0.5 | 620-680 | 22-25 | 8-12 | Martensite + M₆C |
| 0.5-1.0 | 580-640 | 18-22 | 10-15 | Martensite + M₇C₃ |
| 1.0-2.0 | 520-580 | 12-18 | 12-18 | Martensite + M₂₃C₆ |
| 2.0-3.0 | 450-520 | 8-12 | 15-20 | Tempered Martensite |
| 3.0-4.0 | 400-450 | 4-8 | 18-25 | Tempered Martensite |
The hardness gradient from the surface to the fusion line is a critical feature for wear resistance applications, as it provides a hard wear-resistant surface layer supported by a tougher subsurface zone that can absorb impact loads without catastrophic failure.
Mechanical Property Analysis
Hardness Distribution
The surface hardness of TH-950HN deposits typically ranges from 58 HRC to 65 HRC (approximately 620-680 HV), which is significantly higher than the base material hardness of 150-200 HV for structural carbon steel. The high hardness is attributed to the combined effects of martensitic transformation and carbide precipitation. The retained austenite fraction, which typically ranges from 8% to 20% depending on depth, contributes to toughness by providing a mechanism for strain-induced transformation toughening during wear.
Wear Resistance Testing
Abrasive wear testing using ASTM G65 dry sand-rubber wheel method demonstrates that TH-950HN deposits exhibit a wear rate of 0.08-0.12 mm³/N·m, which is approximately 5-8 times better than the uncoated carbon steel substrate. The wear mechanism transitions from abrasive plowing at low loads to microcracking and delamination at higher loads. The carbide particles serve as wear-resistant barriers that resist material removal, while the martensitic matrix provides the necessary toughness to prevent crack propagation.
Impact Resistance
Charpy V-notch impact testing of overlay coupons reveals a transition temperature of approximately 20-30°C, with impact energies of 45-65 J at 25°C for the top layer and 60-85 J for the subsurface layer. The impact resistance is lower than the base material due to the high hardness and brittleness of the martensitic-carbide microstructure, but it remains adequate for most industrial applications where impact loads are moderate.
Process Parameters and Their Effects
The welding process parameters significantly influence the deposit microstructure and properties. The following table summarizes the effects of key parameters:
| Parameter | Low Value | High Value | Effect on Microstructure |
|---|---|---|---|
| Current (A) | 100-120 | 160-180 | Higher current → coarser martensite, larger carbides |
| Travel Speed (mm/min) | 100-150 | 250-350 | Higher speed → finer microstructure, lower hardness |
| Arc Length (mm) | 2-3 | 5-8 | Longer arc → more dilution, softer deposit |
| Lay Width (mm) | 8-10 | 15-20 | Wider lay → lower peak temperature, more retained austenite |
Optimal parameters for TH-950HN typically include a current of 140-160 A, travel speed of 200-250 mm/min, and arc length of 3-5 mm, which produce a deposit with a balanced combination of hardness, toughness, and carbide distribution.
Defect Analysis and Countermeasures
Common defects observed in TH-950HN overlay deposits include:
- Cracking at the fusion line: This is the most critical defect, caused by the mismatch in thermal expansion coefficients between the martensitic deposit and the ferritic base metal. Countermeasures include using a transition layer of lower-hardness material, preheating the base metal to 150-250°C, and controlling the heat input to minimize thermal gradients.
- Porosity: Gas porosity can occur due to moisture in the electrode coating or contamination of the base metal surface. Proper electrode storage and handling, thorough surface preparation, and controlled welding environment are essential.
- Delamination: Insufficient fusion at the fusion line can lead to delamination during service. Ensuring adequate arc cleaning action, proper base metal preparation, and appropriate travel speed are critical.
Engineering Application Considerations
In my experience with hardfacing applications, the TH-950HN electrode system is particularly well-suited for components that experience sliding abrasion with occasional impact, such as conveyor rollers, bucket teeth, and mill liners. The hardness gradient provides an excellent combination of surface wear resistance and subsurface toughness. However, the electrode is not suitable for applications involving severe impact or high-temperature service above 400°C, where the retained austenite may transform and embrittle the microstructure.
For multi-layer builds, I recommend a two-layer approach: the first layer serves as a transition layer with slightly lower hardness to minimize fusion line cracking, while the second layer provides the full hardfacing properties. The interpass temperature should be maintained between 150°C and 250°C to control the cooling rate and minimize residual stresses.
Study Insights and Conclusions
This research provides valuable insight into the microstructure-property relationships governing TH-950HN overlay deposits. The key finding is that the wear resistance is governed by the synergistic interaction between the hard carbide phases and the tough martensitic matrix, with the retained austenite providing additional toughness through strain-induced transformation. The hardness gradient from the surface to the fusion line is not merely a byproduct of the welding process but a beneficial feature that enhances the functional performance of the overlay.
The practical implication is that process engineers should focus on optimizing the heat input to achieve the desired microstructural gradient, rather than simply maximizing hardness. A deposit that is too hard throughout its thickness may be brittle and prone to spalling, while a deposit with an appropriately graded hardness profile will provide superior service life. This understanding should guide procedure qualification and production welding practices.