Mold Tech

Common Defects in Mold Heat Treatment and Their Solutions

• 2024-09-19

(1) Poor Spheroidization Structure

A network, strip, and chain-like carbides may appear in the structure. This is mostly related to poor forging processes or inadequate spheroidizing annealing. Measures include: improving forging processes; using the correct annealing process to eliminate substandard carbides.

(2) Overheating, Overburning, or Coarse Grains after Quenching

Poor spheroidizing annealing; high quenching temperature or prolonged holding time; the workpiece is too close to the electrode or heating element; and large differences in thickness causing local overheating or overburning. Measures include: using the correct annealing process; strict control of quenching process parameters; regular calibration and verification of thermocouples and instruments; ensuring sufficient distance between the workpiece and electrode or heating element.

(3) Low or Uneven Hardness

Serious carbide segregation or poor spheroidization in the original structure; residual decarburization layer on the mold surface or decarburization after quenching; large cross-section of the workpiece with poor hardenability; high quenching temperature leading to increased retained austenite; or low quenching temperature and short heating time resulting in insufficient phase transformation; rapid cooling causing uneven cooling; insufficient tempering, or high tempering temperature leading to non-martensitic structures. Measures: ensure a good preliminary heat treatment structure; strict deoxidation or protection to ensure no decarburization and oxide scale on the surface; use a good hardenability quenching medium; establish reasonable heat treatment quenching process parameters; select a reasonable quenching medium or control operating methods; thorough tempering and prevent the appearance of non-martensitic structures as required.

(4) Decarburization

The salt bath is not deoxidized or deoxidized thoroughly; iron rust is brought into the salt bath by the workpiece or fixture; poor heating or lack of protection in the box furnace. Measures include: regular deoxidation and testing of the salt bath, strict control of the content of barium oxide and other oxides in the furnace; remove as much salt adhering to the workpiece and fixture as possible, and shot blasting or sandblasting of the fixture; use protective measures.

(5) Cracks

Severe network, strip, chain-like carbides in the steel, or microcracks present; large mechanical machining stress or significant plastic deformation; improper heat treatment operations (including fast heating, overly rapid cooling, excessively low temperature when removing the medium during cooling, etc.); overheating or overburning during quenching; complex shape of the workpiece with uneven thickness, causing excessive thermal stress or structural stress; no annealing between two quenchings; insufficient or untimely tempering after quenching; improper grinding after heat treatment; large tensile stress and numerous microcracks in the electric spark machining layer. Measures include: improving forging processes or carrying out correct spheroidizing annealing and normalizing treatment; stress relief annealing after machining; strict implementation of correct heat treatment processes and procedures; preventing local overheating or overburning; taking protective measures or pre-cooling measures; timely and thorough tempering after quenching; determining the correct amount of grinding as required before heat treatment.

(6) Corrosion

Excessive content of carbonate or sulfate in the salt bath; graded quenching or isothermal quenching in nitrate bath at 400-600°C; oxidation of workpieces and fixtures. Measures include: strict control of harmful substances in the salt bath; avoid quenching treatment in nitrate bath; deoxidation of salt bath, shot blasting or sandblasting of fixtures to remove adhering salt residue, etc.

(7) Excessive Brittleness and Breakage

High quenching temperature or prolonged holding time leading to coarse martensite structure, resulting in low impact toughness; insufficient tempering or tempering within the embrittlement zone, low tempering temperature, or short holding time leading to inadequate tempering. Measures include: heating according to the requirements of the quenching process; thorough tempering while avoiding tempering within the embrittlement zone for this type of steel.

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