Heat Treatment Processes & Furnace Types Guide

Batch furnace (Batch Furnaces)

Box furnace / box furnace (Box / Chamber)

The most common batch furnace type, broad specification coverage, suitable for annealing, normalizing, stress relief, tempering, solution treatment and aging.

Pit furnace (Pit)

Vertical arrangement, suitable for hanging long-shaft workpieces for heating, reduce distortion; convenient loading with the furnace cover open, often used for long bars, lead screw, annealing and carburizing of cylindrical parts.

Bell furnace (Bell)

Inner hood / outer-hood structure, bulk material heating, good protective-atmosphere circulation, suitable for coils, annealing of wire coils; multiple positions can rotate, high throughput.

Sealed-quench furnace (Sealed Quench)

Carburizing / quenching / integrated tempering, in-furnace quench oil tank or salt bath, workpiece transfer without oxidation. Suitable for gears, bearing, batch carburizing and quenching of fasteners, etc..

Vacuum furnace (Vacuum)

Heating under vacuum or low-pressure atmosphere, no oxidation or decarburization; hot-wall / cold-wall structure; suitable for tooling, stainless steel, titanium alloy, bright heat treatment of superalloys and vacuum brazing.

Salt-bath furnace (Salt Bath)

Barium chloride / sodium chloride / nitrate medium, fast heat-up, uniform temperature, flexible local heating; suitable for high-speed-steel quenching, austempering, marquenching.

Fluidized-bed furnace (Fluidised Bed)

Airflow suspends particles, heat-transfer coefficient close to salt bath with no pollution; suitable for batch heating of small parts and continuous-furnace preheat zones.

Continuous furnace (Continuous Furnaces)

Kiln (Kilns)

Tunnel kiln ( continuous firing), rotary kiln ( powder material / aggregate), shuttle kiln ( intermittent, movable trolley). Used for ceramic firing, refractory calcination, lime and molding-sand treatment.

Oven (Ovens)

Special-process equipment

Selection points

• Batch size and takt time decide batch or continuous type;
• Max operating temperature and lining grade determine capital cost and energy use;
• Atmosphere requirement ( oxidation / protection / carburizing) determines furnace sealing and auxiliary systems;
• Shop height and charging method ( crane / trolley / roller table) limits furnace geometry.

Core process

Non-ferrous alloy

• Aluminum alloy:T6( solution treatment 525°C + artificial aging 175°C/8h) is a typical condition; annealing O condition softening.
• Copper and brass:Stress-relief annealing 200–300°C; prevention of brass season cracking ( stress relief + slow cool).

Metallurgical basis

TTT the curve describes isothermal transformation (C curve nose 550°C nearby),CCT the curve describes continuous cooling transformation; critical cooling rate Vc determines quench success. Austenite grain size and hardenability (Jominy) is central to grade selection.

Steel-grade database

This siteMaterial databaseCollects critical temperatures of common steel grades, quenchant, tempering curves and isothermal-transformation data, searchable by steel grade.

Process-design principles

• Hardness-toughness trade-off: for each rise in tempering temperature of about 25°C, hardness drops 2–4 HRC;
• Quench-distortion control: pre-cool, marquenching / austempering, press quenching;
• Decarburization and oxidation protection: protective atmosphere, vacuum, salt bath or coating;
• Case-depth profile should transition gradually, avoid carbide network.

Flammable atmosphere

Endothermic atmosphere (Endothermic,RX)

Key process:Carburizing carrier gas ( blended with enrichment gas), carbonitriding, recarburization.Furnace effect:Carbon balance at the steel surface is controlled by carbon potential;CO toxic, explosion-proof.

Exothermic atmosphere (Exothermic,NX)

Oxygen-lean combustion products (N₂ predominant + CO₂/H₂O), used for annealing, positive carbon, copper-brazing protection; low preparation cost, dew-point control removes water.

Dissociated-ammonia atmosphere (AX)

2NH₃ → N₂ + 3H₂(75/25), catalytic cracking; used for stainless-steel annealing, powder metallurgy, bright treatment of low-carbon steel.

Nitrogen-based atmosphere (N₂ + methanol / enrichment gas)

Liquid nitrogen /PSA nitrogen + cracked methanol, flexible ratio, gradually replaces endothermic atmosphere.

Hydrogen (H₂)

Strongly reducing protective atmosphere, used for stainless-steel annealing, powder metallurgy and in-furnace brazing; low density, after a leak it accumulates toward the furnace top, lower flammable limit 4%, explosion-proof design is mandatory ( flame arrester, double-wall pipe, hydrogen-monitor interlock) and pre-ignition purge.

Non-flammable atmosphere

Nitrogen (N₂)

Inert protection, tempering, anti-oxidation annealing; oxygen content controllable <10ppm( high purity).

Argon (Ar)

Vacuum-furnace backfill, protection of titanium and superalloys; density greater than air, good displacement.

Vacuum and low pressure

Vacuum (10⁻¹–10⁻⁴ Pa) or low-pressure inert atmosphere for oxidation-free heating and degassing, suitable for titanium alloys, bright annealing of superalloys and stainless steel; see the vacuum-system topic.

Safety boundary

Carbon-potential control

Use oxygen probe (ZrO₂) oxygen measurement → convert to carbon potential;CO/CO₂ infrared-analysis correction; dew-point meter measures moisture. Target carbon potential 0.9–1.1%( carburizing), deviation ±0.05%.

1. Quenching Theory and the Three Stages

Quenching is steel part cooling from austenitising temperature fast, turning face-centred cubic austenite into body-centred tetragonal martensite – diffusionless shear-type transformation –Cooling rate is the key.。

Critical Temperatures

Critical Cooling Rate (for a Fully Martensitic Structure)

Three Stages of Liquid Quenching

CCT and Hardenability

• Alloy elements (cr, Mo, Ni, Mn) enable pearlite/bainite nose to move right (longer time) to lower critical cooling rate
• Carbon content effect Ms temperature: The higher the carbon, the lower the Ms
• The thicker the hardenability, the better.
• TTT (temperature) maps are used for temperature maintenance;Quenching, please use CCT

2. Quench Severity (Grossmann H-Value)

H-value quantitative medium relative to ideal quenching (H = <unk>) heat-replacement capacity, H is bigger and more intense. To project hardness distribution by Jominy data.

Jominy Equivalence: The range of Jominy with a given H value corresponding to the diameter of the bar can be traced by Lamont figure. Example: <unk>50mm bar in mixed oil (H=0.50) with a heart equivalent Jominy distance of approximately 11mm, surface about 5mm – and thus hardness is predicted from the open Jominy curve.

3. Quench Medium Selection Matrix

4. Oil Quenching

4.1 Classification of Quench Oils

4.2 Commercial Oil Specifications

4.3 Quench Tank Design and Agitation

4.4 Quench Oil Maintenance

Filter:(b) Continuous by-pass filtering (10-25 μm); Magnetic separators decomposers; Bag filters (5-10 μm); Centrifugal separators decomposed of heavy oil sludge.Main causes of deterioration: Hot cracking decomposition (workpiece surface > 300°C), oxidation (high temperature into air), water pollution (heat exchanger leak/cool/wet workpiece), oil sludge (fragmentation carbon residue + scale/oxide scale).

4.5 Oil Quenching Safety

Tank fire disposal:1 Alert evacuation 2 Close the lid 3 Start CO2 flood 4Strictly prohibited Water extinguisher <unk>5 Cut the pumps and mix the power <unk>6 advise the fire department (description quench oil fire) <unk>7 to confirm the re-opening of must not before the cooling is extinguished.

5. High-Pressure Gas Quenching (HPGQ)

HPGQ is a vacuum furnace standard quenching mode: high pressure gas is injected into thermal zone, with a high power fan/blower cycle cooling through heat exchanger. No liquid exposure is no vapour-blanket stage, pure convection cooling.

Gas Characteristics

N2 is the main force (more than 80% HPGQ devices); He is six times higher than cooling capacity 3-4, but the cost limits are applied to critical aviation and tool steel; H2 cooling capacity maximum, subject to ATEX level equipment; Ar is used only in a situation where N2 causes nitrogenization (Ti, r alloy).

Gas–Pressure–Cooling Rate Relationship

System Design Essentials

• Fan/blower: Electrical 75-300 kW (up to 600 kW for large stoves); (b) Centrifugal or axis flow; (b) Top or bottom load; cooling rate for modular velocity control; Overflow 10-30 m/s
• Heat exchanger: Aqueous copper/ stainless steel wings tube, capacity 200-2000 kW; Water flow 5-50 m3/h (water intake 15-25°C); (b) Outside the gas circulation circuits and hot zones; Decreasing capacity, annually check
• Nozzle:(i) 360° full (circle/ cylinder load standard); Orientation nozzle (platform); (b) Re-flow (increased homogeneity per 30-60 seconds); nozzle – Best distance 50-150 mm; Minimum space 25 mm

6. Polymer Quenching (PAG)

PAG polymers use inverse solubility: dissolved in cold water, distilled in heat workpiece surface to form a controlled cooling barrier; The temperature is reduced to the mist point (approximately 74°C) and the polymer is dissolved and recovered. The cooling intensity is controlled by concentration.

Operating Parameters

Commodities: AquaQuench 251 (general steel quenching)/AquaQuench 365 (aluminium solid solution)/Aquatensid N (high performance PAG).Maintenance: Weekly refractometer for concentration; PH to maintain 8.5-9.5 (too low = bacterial breeding); monthly ISO 9950 cooling curve; (a) Refill the tape with concentrate;Prohibited mix different brands PAG(Incompatible polymers can be dissected); annually replaces or cooling curve replaces 20% when deviating from the new liquid.

7. Water and Brine Quenching

Water and salt water provide maximum quench severity (H-Kada 5.0), but distortion cracking risk significant, mainly for low hardenability plain carbon steel, which cannot reach hardness.

Temperature control: Water temperature significant affects intensity (20°C heavy, 60°C much more mild); Production needs to be maintained at 20-30°C with cooling towers or plates; <unk>5°C per unit temperature.Added: Nacl 5-10% (most commonly used, corrosion tanks and workpiece); Naoh 3-10% (vapour blanket better control, pH>12); Na2CO3-5-10% (weak base, corrosion light).

Applicability

• Application: Plain carbon steel (1040-1060), Low hardenability, Sensor quenching
• Edge:>0.50%C medium-carbon steel (cracking risk)
• Avoid: Alloy steel (oil quenching sufficient), complex geometry, thin walls, high-carbon steel

8. Salt-Bath Quenching

The salt bath is maintained at controlled temperature et al. To achieve austempering (bainite) and martempering (low distortion martensite). Workpiece impregnated target temperature smelt and kept to transformation completed or even temperature.

Salt Types

Austempering vs. Conventional Quench-and-Temper

Salt bath maintenance: <unk>5°C (must mix); Monitoring of carbon, scale/oxide scale, chlorination; Water must is zero (water melted salt will be splattered) workpiece washes immediately after bathing (salt is wet corrosion); Bath time 6-12 months.Environmental protection: Nitrate/nitrate is a oxidation agent (UN 1498/1500) that must be disposed of by a certified waster; Cleaning of water is also dangerous (high nitrate). Many applications have been replaced by HPGQ.

9. Cryogenic Treatment

Many tool steels and high-carbon steel Mf are well below 0°C and room temperature quenching will leave a large amount of austenite. Cryogenic treatment completes the transition by cooling to Mf below.

Retained Austenite by Steel Grade (As-Quenched)

Process Sequence

1. Austenite and quenching to room temperature (oil, gas or air)
2. Quenching after 30 minutes cooled to deep cold temperature (Must not first, tempering）
3. Avoid heat shocks by cooling 1-2°C/min
4. Target temperature to maintain the set time
5. Back to room temperature at 1-2°C/min
6. Immediately tempering (tool steel 2 times more)

Main applications: Tool steel (D2/M2/M42) raises hardness and grinding 200-400%; bearing steel (52100) dimensions stabilization; carburising gear (surface retained austenite down to < 5%); Quantities and precision tools; 440C/Surgery equipment to increase corrosivity. Cryogenic treatment (−196°C, 12-36h) can also analyze ultra fine thallium-fe2C carbide, which is more resistant to grinding than a simple austenite conversion (peer review supports 40-30% service life upgrades, depending on steel grade and applications).

10. Quenching Defect Diagnosis

Soft Spots

Quench Cracking

Distortion