Release time：2020-01-16 18:44 Browse：
This is a highly professional and specific question. The carburizing heat treatment of ultra-long screws (e.g., those exceeding 5 meters, even up to 10 meters or more) is a challenging task in the heat treatment industry. The main difficulties lie in achieving uniform carburized case depth, uniform surface hardness, and minimal bending distortion across the entire extreme length.
Below is a detailed process flow, technical difficulties, and solutions for the carburizing heat treatment of ultra-long screws.
Deformation Control: Ultra-long screws can suffer severe sagging and bending deformation under their own weight at high temperatures (~930°C). The rapid cooling during the quenching process after carburizing generates significant thermal and transformational stresses, further exacerbating distortion.
Uniformity Control:
Temperature Uniformity: Maintaining consistent temperature from end to end within a long furnace chamber is challenging. Temperature variations directly cause differences in carburization rates.
Case Depth Uniformity: The flow and circulation of the furnace atmosphere (carbon potential) around an ultra-long workpiece might be uneven, leading to variations in carburized case depth at different parts of the screw.
Cooling Uniformity: During quenching, if the flow rate and temperature of the cooling medium are inconsistent around the ultra-long workpiece, it will result in uneven hardness and additional distortion.
Addressing the above difficulties requires special equipment and process methods for the carburizing heat treatment of ultra-long screws.
Straightening: The raw material must undergo strict straightening first. Better initial straightness allows for better control of final distortion.
Cleaning: Thoroughly clean surface oil stains and rust to ensure uniform carburizing and avoid soft spots.
Anti-Carburizing Treatment: If the threaded part requires carburizing but the shank or mounting areas do not, anti-carburizing treatment (such as applying anti-carburizing coating or copper plating) must be applied to non-carburized areas.
This is the first critical step in controlling deformation.
Special Fixturing: Must use well-designed vertical hanging fixtures.
Hang the ultra-long screws vertically on the rack. This is the most effective way to resist deformation under their own weight.
The rack must be made of heat-resistant alloy (like heat-resistant steel) to withstand long-term high temperatures and the carburizing atmosphere.
Design reasonable support points to avoid introducing new stress deformation from supports at high temperatures.
Uniform Spacing: Maintain sufficient and even distance between screws and between screws and the furnace wall to ensure smooth atmosphere circulation and temperature uniformity.
Equipment Selection: Pit-type gas carburizing furnaces or large vertical carburizing furnaces are the preferred choice. This furnace type is inherently suitable for vertically hanging ultra-long workpieces. The deep chamber, with fans at the top and bottom, facilitates uniform furnace atmosphere and temperature.
Heating Stage: Must adopt a slow heating or stepwise heating strategy. Avoid rapid heating causing excessive internal-external temperature differences and thermal stress deformation.
Boost and Diffuse:
Precisely control the carbon potential and temperature inside the furnace during the carburizing stages (boost and diffuse).
Rely on powerful fans at the top and bottom of the furnace to forcefully circulate the carburizing atmosphere (e.g., propane + carrier gas), ensuring consistency in atmosphere composition and concentration around every part of the ultra-long workpiece.
Modern advanced carburizing control systems (e.g., CP + oxygen probe + CO compensation) are essential for real-time monitoring and adjustment of the furnace conditions.
This is the most critical step for controlling deformation and properties.
Transfer Method: For pit furnaces, the ideal method is for the workpiece to descend rapidly within the furnace directly into the quench oil tank below. This process must be fast (<30 seconds) to avoid cooling and precipitation of pro-eutectoid ferrite.
Quenching Medium: Isothermal quenching oil or high-speed quenching oil is typically used. Its cooling characteristics need to balance quenching severity and deformation control.
Agitation System: The quench tank must be equipped with a uniform, adjustable-speed agitation system. Agitators are usually arranged around the perimeter and bottom of the tank to ensure the ultra-long workpiece is cooled evenly and symmetrically along its entire length. This is paramount for preventing bending distortion.
Direction: Vertical immersion into the oil provides the best cooling symmetry and minimizes deformation.
Cleaning: Clean the quench oil from the workpiece surface after quenching.
Tempering: Tempering must be performed immediately to relieve quenching stresses, stabilize the microstructure, and achieve the required comprehensive mechanical properties. Tempering should also be done in a pit furnace or a box furnace with air circulation, with the workpiece同样 hung vertically to prevent new deformation caused by stress relief during tempering.
Straightening: Even with all measures, micron-level deformation might still exist. For ultra-high precision screws, micro-correction using processes like precision press straightening or hot spotting (heat straightening) might be necessary. Straightening must be done carefully to avoid surface damage or introducing residual stresses.
Inspection:
Straightness: Check straightness over the entire length using precision equipment like optical measuring instruments or laser trackers.
Hardness: Check surface and core hardness. Samples are usually taken from multiple locations like the head, middle, and tail of the screw for testing.
Case Depth: Similarly, take samples from the head, middle, and tail sections to check effective case depth (e.g., at 550 HV) via metallographic or hardness methods, ensuring full-length uniformity.
Microstructure: Check surface martensite, retained austenite grades, and core structure to ensure the absence of undesirable microstructures.
