Stainless steel forgings are widely used in critical applications across industries such as petrochemical, aerospace, power generation, food processing, and marine engineering. These components are known for their superior strength, corrosion resistance, and durability under extreme conditions. However, the reliability of stainless steel forgings depends not only on their external dimensions but also on their internal quality.
Internal defects like cracks, porosity, inclusions, or segregation—if left undetected—can lead to catastrophic failures in service. Therefore, internal quality inspection is a vital step in the production and quality assurance process. It ensures that forged products meet the mechanical, metallurgical, and safety requirements demanded by international standards and end users.
This article explores the key methods and importance of internal quality inspection in stainless steel forgings, highlighting industry best practices and quality control principles followed by trusted suppliers like sakysteel.
Even when the surface appears flawless, a stainless steel forging may contain hidden flaws that compromise:
Mechanical strength
Fatigue resistance
Corrosion behavior
Weldability
Dimensional stability
These internal flaws often originate from raw material inconsistencies, improper forging parameters, or inadequate heat treatment.
By performing internal inspections, manufacturers can:
Ensure product integrity
Reduce failure rates
Meet third-party inspection requirements
Improve traceability and documentation
Before diving into inspection methods, it’s important to understand the types of internal defects commonly found in forgings:
Internal cracks
Non-metallic inclusions
Shrinkage cavities or voids
Porosity due to gas entrapment
Segregation of alloying elements
Laminar defects due to rolling or forging laps
Identifying and eliminating these defects is essential, especially for components used in pressure vessels, turbines, flanges, pumps, and structural assemblies.
Overview:
Ultrasonic testing is the most commonly used non-destructive testing (NDT) method to inspect the internal structure of stainless steel forgings.
How it works:
High-frequency sound waves are introduced into the forging. If there is a discontinuity, such as a crack or void, part of the sound wave reflects back and is captured by the probe.
Benefits:
Detects internal defects up to several inches deep
Suitable for forgings of various shapes and sizes
Fast and cost-effective
Applications:
Used for shafts, disks, rings, and flanges where structural integrity is critical.
At sakysteel, UT is performed using calibrated instruments and certified inspectors to ensure every defect is mapped and recorded.
Overview:
Also known as X-ray or gamma ray inspection, radiographic testing produces an image of the forging’s internal structure.
How it works:
A radiation source is placed on one side of the forging, and a film or digital detector is placed on the other. Variations in thickness and density reveal defects.
Advantages:
Visual image of internal flaws
High sensitivity for porosity and shrinkage cavities
Permanent inspection record
Limitations:
More expensive than UT
Requires safety precautions due to radiation
Not ideal for very thick sections
Used selectively at sakysteel for high-risk or highly engineered components.
Overview:
This method involves cutting, polishing, and etching a sample section of the forging to observe the microstructure under a microscope.
What it reveals:
Grain flow and size
Inclusions
Micro-cracks
Heat treatment effectiveness
Phase distribution (austenite, ferrite, martensite)
Tools used:
Optical microscope
Scanning electron microscope (SEM)
Importance:
Essential for understanding material behavior and predicting long-term performance.
Used at sakysteel during product development or customer qualification processes.
Though primarily surface methods, they can help detect subsurface flaws near the outer layers of stainless steel forgings.
Dye Penetrant Testing (DPT): Reveals open surface cracks using dye and developer
Magnetic Particle Testing (MPI): Used only for magnetic stainless steels like 410 or 420
These methods often complement deeper internal inspections.
Overview:
Verifying internal consistency of chemical elements is critical for corrosion resistance and mechanical properties.
Methods used:
Optical Emission Spectroscopy (OES)
X-ray Fluorescence (XRF)
Glow Discharge Spectroscopy (GDS)
Focus areas:
Detecting segregation in larger forgings
Ensuring consistency from surface to core
sakysteel performs composition checks before and after forging for full traceability.
Purpose:
To assess uniformity of mechanical properties throughout the forging.
How it’s done:
Sectioned samples are tested at various depths
Hardness readings (Brinell, Rockwell, or Vickers) are mapped
Helps detect improper heat treatment or cooling effects in large forgings.
Internal inspection results must comply with relevant industry standards. Common standards referenced include:
ASTM A388 – UT for steel forgings
EN 10228 – NDT of steel forgings
ASME Section VIII – Pressure vessel forging standards
API 6A / 6D – Oil and gas industry standards
Acceptance levels depend on customer requirements, criticality of the part, and application risks.
At sakysteel, all internal quality inspections are documented as part of a quality control package, including:
Ultrasonic test reports
Radiographic film or digital records
Microstructure photos
Hardness and chemical composition charts
Heat number traceability
Mill Test Certificates (MTCs)
EN 10204 3.1 or 3.2 certification (as required)
This ensures customers receive full transparency and confidence in product reliability.
Internal quality inspections are especially crucial for:
Pressure-rated stainless flanges
Pump housings and impellers
Turbine disks and rotors
Nuclear and aerospace parts
Subsea and offshore components
In these applications, undetected flaws can lead to costly downtime or life-threatening failures.
Internal quality inspection of stainless steel forgings is a non-negotiable aspect of modern manufacturing. Through methods like ultrasonic testing, radiographic analysis, metallography, and hardness mapping, manufacturers can detect internal issues before they become operational problems.
With increasing demand for high-performance forged components in critical industries, quality assurance practices must be thorough, standardized, and traceable.
sakysteel takes pride in offering stainless steel forgings that are not only visually perfect but internally sound. From raw material inspection to final certification, every product is evaluated using advanced inspection techniques and delivered with complete quality documentation.