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QPQ Salt Bath Heat Treatment Furnace
COOLDO FERRITIC NITROCARBURIZING EQUIPMENT
QPQ Salt Bath Nitriding Furnace and Complete Heat Treatment Line
COOLDO QPQ equipment integrates cleaning, controlled preheating, salt bath nitrocarburizing, oxidation, polishing, re-oxidation and sealing for ferrous-metal parts. Standard project references cover Φ600×1000, Φ800×1200 and Φ900×1500 mm work zones, with furnace, cleaning, handling and programmable control systems engineered as one production line.
- 3 Reference Sizes
- Φ600×1000 to Φ900×1500 mm
- 3 Main Furnaces
- Preheating, nitriding and oxidation
- 2 Control Zones
- 900×1500 technical-agreement reference
- 380 V / 3 Phase
- Reference supply; other voltages by project
What Is QPQ Salt Bath Heat Treatment?
QPQ means Quench–Polish–Quench. It is a ferritic nitrocarburizing and post-oxidation process for ferrous parts. Nitrogen and carbon diffuse into the surface at a temperature below the steel transformation range, followed by an oxidizing treatment, controlled polishing and a second oxidation or sealing stage. The process is used to improve surface hardness, wear behavior, corrosion resistance and friction performance while limiting dimensional change.
The result is a compound layer over a diffusion zone, with a dark oxide finish formed during the oxidation stages. Final layer depth, hardness, appearance and corrosion performance depend on the steel grade, starting condition, salt chemistry, actual temperature, treatment time, polishing method and quality control.
- Process family
- Salt bath ferritic nitrocarburizing with post-oxidation.
- Typical nitriding reference
- Approximately 570±10°C; validated by material and recipe.
- Suitable materials
- Qualified carbon, alloy and tool steels, cast iron and iron-based PM parts.
- Main outputs
- Improved surface wear, corrosion and friction performance with low distortion.
Important distinction: the 650°C and 950°C values in the equipment agreement are furnace design ratings. They are not the normal set temperatures for every QPQ process step. Actual recipes must be qualified for the part material, salt system and required layer.
Complete QPQ Salt Bath Furnace Line
The supplied scope is configured around production capacity, basket size, part geometry, cleaning standard and required finishing route.
Ultrasonic Cleaning
Removes oil, machining residue and surface contamination before heat treatment.
Rinse and Drying
Prevents cleaner carryover and moisture entering the heated process line.
Circulation Preheating Furnace
Preheats the load and can also support specified tempering operations.
Nitriding Salt Bath Furnace
Provides controlled salt temperature for the main nitrogen-and-carbon diffusion stage.
Oxidation Salt Bath Furnace
Supports the first quench/oxidation and the controlled second oxidation stage.
Cooling and Rinse Tanks
Controls cooling and removes residual salt between process stages.
Polishing Station
Adjusts surface roughness and removes the porous outer surface as required.
Sealing Tanks
Water-soluble or oil-based sealing is selected for the finish and service condition.
Fixtures and Baskets
Part-specific tooling supports drainage, circulation, safe lifting and minimal contact marking.
Testing and Control
Temperature records, recipe storage and metallurgical inspection support process traceability.
Three Main Furnace Functions
- Circulation preheating furnace: insulated all-fiber chamber, resistance heating, two control zones and a circulation fan for load preheating and temperature equalization.
- Nitriding furnace: sealed salt pot, 316L reference construction, salt-temperature thermocouples, pneumatic lid lift and fume connection.
- Oxidation furnace: oxidation salt pot, slag collection, temperature sensing, pneumatic lid lift and controlled post-treatment.
- Programmable control: temperature display, target setting, PID output, process timing, alarm status and recipe/record functions.
QPQ Quench–Polish–Quench Process Flow
This engineering sequence is adjusted after trials; temperatures and times are not copied blindly from another part.
Incoming Inspection
Confirm steel grade, heat-treatment condition, dimensions, surface finish and prohibited contamination.
Degrease and Clean
Use the specified water-based cleaner and ultrasonic action to remove oil and residues.
Rinse and Dry
Remove cleaner carryover and dry the load completely before preheating.
Load and Preheat
Place parts on suitable tooling and preheat to the qualified recipe for thermal equalization.
Salt Bath Nitriding
Treat in the controlled nitrocarburizing bath; approximately 570±10°C is a common reference, not a universal recipe.
First Oxidation
Transfer to the oxidation bath to form the initial oxide layer and control post-nitriding cooling.
Cool, Rinse and Dry
Remove residual salt using the specified rinse sequence before mechanical finishing.
Controlled Polishing
Polish by the approved method to achieve the required surface roughness without damaging the compound layer.
Second Oxidation / Seal
Re-oxidize and apply the selected water- or oil-based sealing treatment when specified.
Final Inspection
Check appearance, dimensions, surface hardness, layer structure and corrosion requirements to the part control plan.
Process control: recipe qualification should define bath composition control, salt analysis, part loading, preheat, transfer time, nitriding, oxidation, polishing, rinsing, sealing and inspection. COOLDO supplies the equipment; the production recipe must be confirmed with the approved salt supplier and the customer's metallurgical requirements.
QPQ Furnace Line Model and Size Comparison
These reference configurations are taken from the supplied 600×1000, 800×1200 and 900×1500 quotations. Prices and commercial terms are intentionally excluded from this product page.
| Reference line | Preheating furnace | Nitriding furnace | Oxidation furnace | Cleaning / project reference |
|---|---|---|---|---|
| Φ600×1000 mm | QPQ3-35-6 Φ600×1000 mm | QPQ3-50-9 Φ600×1000 mm | QPQ3-45-9 Φ700×1000 mm | Ultrasonic: 1000×1000×1200 mm 304 rinse tank: 1200×1200×1500 mm |
| Φ800×1200 mm | QPQ3-60-6 Φ800×1200 mm | QPQ3-90-9 Φ800×1200 mm | QPQ3-90-9 Φ900×1200 mm | Ultrasonic: 1000×1000×1500 mm Rinse tank: 1200×1200×1500 mm |
| Φ900×1500 mm | QPQ3-80-6 80 kW Φ900×1500 mm | QPQ3-100-9 100 kW Φ900×1500 mm 316L salt pot | QPQ3-90-9 90 kW Φ900×1500 mm | Example high-throughput project: 2 preheating + 4 nitriding + 2 oxidation furnaces |
The 2+4+2 quantity is the configuration recorded in the supplied Φ900×1500 quotation, not a compulsory quantity for every QPQ line. Final furnace quantities are calculated from treatment time, transfer time, basket load and required daily throughput. The oxidation vessel may be larger than the other work zones in some layouts.
Construction and Control Features
Key construction details preserved from the Φ900×1500 technical agreement.
Rigid Steel Frame
Welded steel-plate and structural-section frame supports the furnace, salt pot, lid mechanism and production loads.
All-Fiber Insulation
Compressed ceramic-fiber lining with a reference insulation thickness of at least 350 mm reduces stored heat and external heat loss.
Resistance Heating
0Cr25Al5 resistance elements are divided into two heating zones and matched to the control circuits.
Salt Pot Construction
The agreement specifies 12 mm 316L for the nitriding pot and 12 mm 304 stainless steel for the oxidation pot.
Pneumatic Lid Lift
Cylinder-operated lids provide controlled opening; the reference agreement lists approximately 30-70 mm/s lift speed.
Programmable Monitoring
Touchscreen control supports temperature display, setpoint, PID output, timing, alarms and process record functions.
Materials for QPQ Treatment
- Low-Carbon Steel
- Structural Steel
- Alloy Steel
- Tool Steel
- High-Speed Steel
- Selected Stainless Steel
- Cast Iron
- Pure Iron
- Iron-Based Powder Metallurgy
Suitability and results must be verified for the exact grade, previous heat treatment and dimensional tolerance. Non-ferrous metals are not the normal application of this ferritic nitrocarburizing line.
Typical QPQ Parts
- Gears, shafts, pins and bearing components
- Crankshafts, camshafts and valve parts
- Shock-absorber and hydraulic piston rods
- Injection, die-casting and extrusion molds
- Cutting tools and machine-tool components
- Construction and agricultural machinery parts
- Printing, textile and power-tool components
- Valves, sealing parts and precision hardware
Salt Bath Safety and Environmental Engineering
Salt and Chemical Control
Use only approved salt chemistry with supplier safety data, controlled additions, bath analysis, labeling, secure storage and trained operators. Do not introduce wet parts or incompatible materials into molten salt.
Fume Capture
Local exhaust, covered transfer routes and appropriate gas treatment are engineered for the selected chemistry and local emission requirements.
Water and Waste Management
Rinse water, drag-out salt, sludge and spent salt require collection and treatment under local environmental rules. "Pollution-free" should not be used as an unconditional claim.
Interlocks and Handling
Temperature limits, power protection, lid controls, lifting fixtures, drainage, emergency procedures and PPE are incorporated into the operating system and training plan.
QPQ Salt Bath Furnace FAQs
Technical answers for QPQ nitriding equipment, process planning and line selection.
What does QPQ mean in heat treatment?
QPQ means Quench–Polish–Quench. A ferritic nitrocarburizing stage is followed by oxidation, controlled polishing and a second oxidation or sealing stage to develop the required surface properties and dark finish.
What is a QPQ salt bath furnace line?
It is a coordinated production line containing cleaning and rinse equipment, a circulation preheating furnace, nitriding salt bath furnace, oxidation salt bath furnace, cooling, polishing, sealing, tooling, fume control and programmable process controls.
What is the normal QPQ nitriding temperature?
Approximately 570±10°C is a common process reference. The exact setpoint and time must be qualified for the steel grade, salt chemistry, starting microstructure and required compound/diffusion layer.
Why are some furnaces rated at 950°C if QPQ runs near 570°C?
The 950°C value in the technical agreement is an equipment design rating for the nitriding and oxidation furnaces; it is not the normal QPQ recipe temperature. Equipment capability and actual process setpoint are different specifications.
Which QPQ furnace sizes are available?
The supplied documents contain reference lines for Φ600×1000, Φ800×1200 and Φ900×1500 mm. Custom sizes are engineered from basket dimensions, load weight, production rate, tooling clearance and plant layout.
What equipment is included in a complete QPQ line?
A typical scope includes ultrasonic cleaning, rinse and cooling tanks, preheating furnace, nitriding furnace, oxidation furnace, polishing and sealing stations, fixtures, slag tools, thermocouples, controls, fume extraction interfaces and laboratory inspection facilities.
Which metals can be QPQ treated?
The process is intended mainly for ferrous materials such as carbon and alloy steels, tool steel, selected stainless grades, cast iron and iron-based powder-metallurgy parts. Each grade requires trials and metallurgical approval.
What surface improvements can QPQ provide?
A qualified process can improve surface hardness, wear behavior, corrosion resistance, fatigue behavior and friction performance while limiting distortion. Numeric results cannot be guaranteed without a material, geometry and test specification.
Does QPQ cause part distortion?
QPQ operates below conventional austenitizing temperatures, so distortion is generally lower than carburizing and quench hardening. Thin, asymmetric or highly stressed parts can still move, so dimensional trials and correct fixturing are required.
Why is ultrasonic cleaning important before QPQ?
Oil, polishing compound and machining residue can contaminate the bath and cause an uneven surface. A controlled cleaning, rinsing and drying sequence is essential before preheating and salt treatment.
What is checked after QPQ treatment?
Inspection can include appearance, dimensions, surface hardness, compound-layer thickness, diffusion depth, microstructure, roughness, corrosion testing and adhesion or functional testing required by the customer drawing.
Can the QPQ line record temperature and recipes?
Yes. The reference control system supports temperature display and setpoint, PID output, timing, alarms and programmable monitoring. Networked records and additional traceability can be configured in the final control specification.
Is QPQ an environmentally friendly process?
It can replace or reduce some higher-temperature or electroplating operations, but it still uses process salts and produces drag-out, rinse water, fumes and waste. Safe chemical management, capture and compliant treatment are mandatory.
What information is needed for a QPQ furnace quotation?
Send part material, heat-treatment condition, maximum dimensions and weight, basket size, load weight, daily production, required layer/hardness/corrosion results, process route, available voltage, plant layout, exhaust and wastewater conditions, and destination standards.
Need a QPQ Line for Your Part and Production Capacity?
Send part drawings, steel grade, basket size, daily output and required surface results for an engineered QPQ furnace-line proposal.