Common quality problems and solutions in steel forging production

Common quality problems and solutions in steel forging production

In the production process of steel forging parts, quality control is a key link to ensure product performance and safety. Even with the most advanced equipment and processes, various quality problems will inevitably occur during the forging process due to the complexity of material properties, process parameters and environmental factors. These problems not only affect the appearance and dimensional accuracy of the product, but are also likely to have a serious impact on the mechanical properties and service life of the parts.

This section will systematically analyze the seven most common quality problems in the steel forging process, including surface cracks, internal inclusions, dimensional tolerances, coarse structure, uneven hardness, folding defects and excessive residual stress. For each type of problem, we will deeply analyze its generation mechanism, analyze the root cause from multiple dimensions such as raw materials, process parameters, equipment status, and provide effective solutions that have been verified in practice.

1. Surface crack problem

Typical manifestation: longitudinal or reticular cracks appear on the surface of forgings

Root cause analysis:

Heating stage: too fast heating rate leads to thermal stress cracks (especially high alloy steel)

Forging stage: hammering too hard or final forging temperature is lower than the minimum forging temperature allowed by the material

Cooling stage: improper cooling rate produces quenching cracks

Solution:

Develop a step-type heating curve and use a preheating stage for high-carbon high-alloy steel

Install an infrared thermometer to monitor the temperature of the forging in real time to ensure that the final forging temperature is higher than the Ar3 line

Use slow cooling methods such as pit cooling or furnace cooling for crack-prone materials

1. Internal inclusion defects

Typical manifestation: ultrasonic testing finds internal point or strip defects

Root cause analysis:

Raw materials: non-metallic inclusions formed by poor deoxidation during steelmaking

Process problem: insufficient forging ratio leads to insufficient crushing of the cast structure

Pollution problem: oxide scale falls off in the heating furnace and mixes into the forgings

Solution:

Purchase in accordance with ASTM A788 standard high-quality forging billet

Ensure the total forging ratio ≥4:1 (important parts ≥6:1)

Use protective atmosphere heating furnace and clean the furnace regularly

1. Dimension out-of-tolerance problem

Typical manifestation: key dimensions exceed the tolerance requirements of the drawing

Root cause analysis:

Mold factor: wear causes cavity size changes (on average, every 1,000 pieces need to be tested)

Temperature factor: uneven billet temperature causes deformation resistance differences

Equipment factor: press parallelism deviation or slider guide gap is too large

Solution:

Establish a mold life management system and set a mandatory replacement cycle

Use a multi-zone temperature-controlled heating furnace with a temperature difference controlled within ±15°C

Check the geometric accuracy of the equipment daily and perform preventive maintenance

1. Coarse organization problem

Typical manifestation: metallographic detection found that the grain size exceeded the standard (coarser than ASTM Level 5)

Root cause analysis:

Overheating problem: Heating temperature exceeds Ac3 line too much

Deformation problem: Insufficient deformation in the last fire (<15%)

Cooling problem: Too slow cooling speed in high temperature stage

Solution:

Set temperature monitoring alarm to prevent furnace temperature from getting out of control

Ensure sufficient deformation in the final forging stage (20-30% recommended)

Use accelerated cooling measures such as spray cooling for important parts

1. Uneven hardness problem

Typical manifestation: Hardness fluctuation of the same forging exceeds HRC3 degrees

Root cause analysis:

Quenching factor: Uneven stirring of cooling medium

Composition segregation: Uneven distribution of alloy elements in the material

Decarburization problem: Loss of surface carbon element during heating

Solution:

Install propeller agitator in quenching tank, flow rate ≥0.8m/s

Use high-quality steel such as ESR (electroslag remelting)

Control carbon potential in heating furnace, or use vacuum heat treatment

1. Folding defect

Typical manifestation: Metal overlap on metal forging surface Traces

Root cause analysis:

Mold design: fillet radius is too small or draft angle is insufficient

Process parameters: each press amount is too large (>30%)

Operational problem: blank placement position is offset

Solution:

The fillet radius of key parts of the mold is ≥5mm

Use multi-pass small deformation process (15-25% per pass)

Use a manipulator to ensure accurate positioning of the blank

1. Excessive residual stress

Typical manifestation: parts deform after machining

Root cause analysis:

Improper cooling: air cooling speed is too fast

Uneven deformation: asymmetric forging leads to unbalanced stress distribution

Insufficient heat treatment: insufficient tempering

Solution:

Implement stress relief annealing (550-650℃) for large forgings

Use symmetric forging process design

Tempering and holding time is calculated based on the maximum cross-section 1.5h/25mm

By establishing a rapid response mechanism for the above quality problems, the scrap rate can be significantly reduced (the empirical value can be reduced to less than 0.5%). It is recommended that enterprises be equipped with basic testing equipment such as metallographic laboratories and portable hardness testers to achieve early detection and timely correction of problems.