Surfacing Technology for Guide Rails of Steel Ingot Induction Heating Furnace

Literature Overview

This paper, authored by Du Qing from the Process Department of Hangzhou Switchgear Factory, was published in the journal Industrial Heating in 1997 (Vol. 26, No. 4, pp. 42). The study addresses a practical engineering problem in medium-frequency induction heating furnaces used for steel ingot heat treatment: the degradation of guide rails due to inadequate surface hardening and wear resistance. The original guide rail material—typically austenitic or non-magnetic stainless steel—was left untreated on the surface, resulting in poor wear resistance and insufficient heat hardness under the harsh operating conditions of the furnace.

Problem Analysis and Root Cause Identification

The guide rails in medium-frequency induction heating furnaces serve a critical mechanical function: they guide and support steel ingots during the heating process. The operating environment is demanding, involving sustained high temperatures, mechanical sliding contact, and sometimes abrasive particles from scale and oxide formation. The untreated austenitic stainless steel surface, while offering good corrosion resistance and non-magnetic properties, suffers from the following deficiencies:

Low hardness: Austenitic stainless steels in the as-rolled or annealed condition typically exhibit hardness values of 150–200 HV, which is insufficient for resisting abrasive wear from steel ingot surfaces.
Poor heat hardness: At elevated temperatures (above 400 °C), the hardness of austenitic stainless steels drops significantly due to solid-solution softening, accelerating wear.
Surface quality mismatch: The front-end cutting process for ingot preparation produces a bevel angle of 3°–5° with curled burrs, which exceeds the ideal 2° tolerance and introduces additional mechanical stress concentrations on the guide rail surface.

Surfacing Technology Solutions

The study proposes surfacing (overlay welding) as the remedial approach. The core idea is to deposit a hard, wear-resistant, and heat-resistant alloy layer on the guide rail surface while maintaining the non-magnetic property required for induction heating compatibility.

Key Technical Considerations

Parameter	Requirement	Rationale
Surface hardness	≥ 40 HRC	Resist abrasive wear from steel ingots
Heat hardness at 600 °C	Retain ≥ 30 HRC	Maintain wear resistance under furnace temperatures
Magnetic permeability	Non-magnetic	Avoid interference with induction heating field
Coating thickness	3–5 mm	Balance wear life against distortion control
Dilution rate	< 30%	Preserve the hardness of the overlay alloy

Welding Process Selection

The surfacing process for this application typically involves GTAW (Tungsten Inert Gas Welding) or SMAW (Shielded Metal Arc Welding) with appropriate filler metals. The selection of filler metal is critical:

Ni-based alloys (e.g., Ni-20Cr, Ni-30Cr) offer excellent heat hardness and non-magnetic properties but are expensive.
Cr-Mo-V alloy steels (e.g., Cr12MoV-based) provide high hardness and good wear resistance but may introduce magnetic properties.
Austenitic stainless steels with carbide-forming elements (e.g., 310 with Ti or Nb additions) can balance non-magnetic requirements with improved wear resistance.

Process Parameters and Defect Prevention

The following process window is recommended for the surfacing operation:

Preheating temperature: 150–200 °C to reduce residual stress and prevent cold cracking, particularly when surfacing on carbon steel guide rail substrates.
Interpass temperature: Maintain below 250 °C to control grain growth and minimize thermal distortion.
Welding current: Adjust according to the specific filler metal; typically 100–180 A for GTAW with a 2.5–3.2 mm diameter electrode.
Travel speed: 40–80 mm/min to ensure adequate penetration without excessive dilution.

Common defects in this surfacing application include:

Defect	Cause	Countermeasure
Cracking	High dilution with carbon steel base; rapid cooling	Increase preheat; use Ni-based transition layer
Porosity	Inadequate gas shielding; contamination	Ensure clean surface; use pure argon shielding
Excessive dilution	Too high welding current; insufficient travel speed	Reduce current; increase travel speed
Distortion	Thermal input too high; constrained geometry	Use back-up copper plate; control interpass temperature

Engineering Practice Insights

From an engineering practice perspective, this study highlights an important principle: the base material's inherent properties may be adequate for one function (e.g., non-magnetic compatibility) but inadequate for another (e.g., wear resistance). Surfacing provides a cost-effective solution that decouples these requirements. The choice of filler metal must be evaluated not only on its as-deposited properties but also on its behavior under the actual service temperature and the dilution interaction with the substrate.

The requirement for a 2° bevel angle on ingot end faces, which is difficult to achieve in mass production (3°–5° with burrs), suggests that the surfacing layer must also accommodate geometric imperfections. A thicker overlay (4–5 mm) with adequate hardness can compensate for the increased contact stress at the burr region. However, this must be balanced against the risk of cracking and distortion, which argues for a multi-pass surfacing strategy with a transition layer followed by the hard alloy topcoat.

Study Reflections

This 1997 paper, while technically straightforward, embodies a timeless engineering philosophy: understand the failure mode, identify the root cause, and apply the minimum effective solution. The guide rail problem is not unique to induction heating furnaces; similar issues arise in continuous casting machine rollers, reheating furnace skid bars, and rolling mill guide shoes. The surfacing approach described here can be directly transferred to these applications with appropriate adaptation of filler metal selection and process parameters.