Microstructural Analysis of CO2 Shielded Weld Overlay on Press Roller Shaft

Literature Overview

This 2002 publication from the Industrial Testing Experimental Center of Guangxi University investigates the microstructural evolution in CO2 gas shielded arc weld overlay deposits applied to press roller shafts. Press roller shafts in sugar cane processing and metal rolling industries are subjected to severe abrasive and adhesive wear conditions, necessitating hardfacing overlay systems. The study provides fundamental metallurgical insights into the phase transformations and mechanical behavior of the overlay layer under CO2 shielding conditions.

Core Technical Analysis

The CO2 gas shielded arc welding process (GMAW with CO2 shielding) is widely adopted for overlay applications due to its high deposition rate, good penetration characteristics, and relatively low equipment cost. However, the use of pure CO2 as a shielding gas introduces unique metallurgical challenges including carbon pickup, martensite formation, and potential brittleness in the deposited microstructure.

Microstructural Characteristics

Microstructural Feature	Observation	Implication
Primary phase	High-carbon martensite	High hardness (HRC 45–55) but potential brittleness
Retained austenite	10–25 vol%	Provides toughness and reduces cracking tendency
Cementite (Fe3C)	Dispersed in matrix	Contributes to wear resistance
Grain boundary carbides	Cr23C6, Cr7C3	Network formation can reduce toughness
Dilution zone	0.5–1.5 mm transition	Gradient in carbon content and hardness

Process Parameters Investigated

Parameter	Value	Effect on Microstructure
Shielding gas	CO2 (99.99%)	High carbon activity, promotes martensite
Wire composition	Fe-Cr-C-Ni hardfacing	Controls dilution and phase stability
Welding current	200–280 A	Higher current increases dilution
Travel speed	200–350 mm/min	Slower speed increases heat input and grain coarsening
Wire diameter	1.2 mm	Standard for overlay applications
Number of passes	2–3 layers	Multi-pass improves bonding and reduces dilution

Mechanical Properties and Wear Performance

The overlay deposits exhibit a hardness range of 48–58 HRC, significantly exceeding the base steel hardness of approximately 25–30 HRC. The retained austenite fraction plays a dual role: it provides strain-induced hardening during service (TRIP effect) and simultaneously acts as a crack-arresting phase that prevents catastrophic brittle failure. The wear test results demonstrate that the CO2-shielded overlay achieves 3–5 times the wear life of the uncladded base material in sugar cane press roller service.

FMEA Analysis of Overlay Failure Modes

Failure Mode	Severity	Occurrence	Detection	Risk Priority	Mitigation
Spalling due to brittle fracture	9	4	5	180	Increase Ni content, reduce carbon
Cracking at overlay boundary	8	3	4	96	Control pre-heat and interpass temperature
Excessive dilution	6	5	3	90	Reduce current, increase travel speed
Porosity	5	4	3	60	Ensure gas flow rate and joint fit-up

Study Insights and Implications

The fundamental contribution of this research is the establishment of a clear relationship between the CO2 shielding environment and the resulting microstructure-property correlation in hardfacing deposits. The presence of retained austenite, while generally undesirable in structural welds, is beneficial in this application as it provides a balance between hardness and toughness. The study highlights that the carbon activity of CO2 shielding gas must be carefully managed through wire composition design rather than gas blending. For engineers specifying overlay systems for press roller applications, the key takeaway is that CO2-shielded GMAW is a viable and economical option provided that the wire composition is optimized to maintain sufficient retained austenite (15–25%) for toughness while achieving target hardness above 48 HRC. The dilution zone analysis further underscores the importance of multi-pass welding strategies to minimize base metal dilution and maintain consistent overlay properties across the full thickness.