Quality Assurance in Heat Treatment
Ensuring that parts meet specification is a critical function, combining rigorous testing, meticulous process control, and accurate documentation.
1. Documentation
Standard Operating Procedures (SOPs) and Work Instructions provide clear, step-by-step guidance for every process, ensuring consistency and repeatability.
2. Traceability
A 'job traveler' or 'router' document accompanies parts through the entire process, recording every step, operator, furnace, and test result for full traceability.
3. Verification
Includes in-process checks (e.g., temperature monitoring) and final inspection (e.g., hardness testing, case depth analysis) to verify that all customer requirements have been met.
4. Calibration
All measurement and testing equipment, from furnace thermocouples to hardness testers, must be regularly calibrated against traceable standards to ensure accuracy.
Common Methods:
- Rockwell (HRC, HRB)
- Brinell (HBW)
- Vickers (HV)
- Microhardness (Knoop/Vickers)
Common Methods:
- Charpy V-Notch
- Izod
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Sectioning
Cut a small, representative piece from the component using an abrasive cut-off saw with coolant to prevent overheating and altering the microstructure.
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Mounting
The sample is embedded in a polymer resin (e.g., epoxy) to make it easier to handle and hold flat during polishing. For **cold mounting**, a two-part liquid resin is mixed and poured into a mold containing the sample, then allowed to cure at room temperature.
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Grinding & Polishing
The mounted sample is ground with progressively finer silicon carbide papers (e.g., 240, 400, 600, 1200 grit) to achieve a flat surface. It is then polished on cloths with diamond slurries (e.g., 9, 3, 1 micron) to produce a mirror-like, scratch-free finish.
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Etching
The polished surface is chemically etched to reveal the microstructure. For steels, a common etchant is **Nital** (2-5% nitric acid in ethanol). The sample is swabbed or immersed for a few seconds, which corrodes different phases and grain boundaries at different rates, creating visual contrast under a microscope.
Safety Note: Always handle etchants in a fume hood with appropriate PPE (gloves, goggles, lab coat).
Microstructure Analysis
Visual examination of the polished and etched surface of a material under a microscope to identify phases (e.g., martensite, bainite, ferrite) and detect defects.
Grain Size Measurement
Determining the average grain size of the material, which significantly impacts mechanical properties like strength and toughness. (e.g., per ASTM E112).
- Hardness Testers (Rockwell, Brinell, Vickers)
- Metallurgical Microscopes with imaging software
- Cut-off wheels, mounting presses, and polishing machines for sample prep
- Spectrometers for chemical analysis (verifying alloy composition)
- Tensile and Impact testing machines
1. Visual Method
A cross-section of the part is polished and etched with an appropriate acid (like Nital). The hardened case will appear as a distinct, darker layer, which can be measured with a calibrated microscope or optical comparator.
2. Microhardness Traverse Method
This is the most accurate method. A microhardness tester (Vickers or Knoop) is used to make a series of indentations from the surface towards the core. The "effective case depth" is defined as the depth at which the hardness drops to a specified value (e.g., 50 HRC or its Vickers equivalent).
Depth (mm)
Hardness (HV/HRC)
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Identification & Segregation
When a non-conforming part is identified (e.g., wrong hardness, cracked, wrong case depth), it must be immediately segregated from the good parts and clearly labeled to prevent it from being shipped accidentally.
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Disposition
The non-conforming product is reviewed by a Material Review Board (MRB), which typically includes quality, engineering, and production representatives. The disposition can be: Rework (re-heat treat), Use-As-Is (if the deviation is minor and acceptable to the customer), or Scrap.
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Root Cause Analysis
This is the most critical step. The team investigates to find the true cause of the problem using tools like the "5 Whys" or a Fishbone (Ishikawa) Diagram. The goal is to find the systemic issue, not just blame an operator.
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Corrective & Preventive Action (CAPA)
Based on the root cause, a corrective action is implemented to fix the immediate problem (e.g., recalibrate a furnace). A preventive action is also put in place to ensure it never happens again (e.g., change the calibration schedule from yearly to quarterly).
Failure Analysis & Root Cause Investigation
Systematic approaches to understanding why a failure occurred and preventing it from happening again.
The problem or "effect" is placed at the head of the "fish". The major categories of causes are the main bones, with specific causes branching off them. The "6 Ms" are a common starting point for categories in manufacturing:
Manpower
Method
Machine
Material
Measurement
Mother Nature (Environment)
- Executive Summary: A brief, high-level overview of the failure and the key findings.
- Background & Part Information: Describe the part, its material, specifications, and service history.
- Investigation Procedure: Detail the tests performed (e.g., visual inspection, hardness testing, metallography, chemical analysis).
- Findings & Observations: Present the data and results from the tests in a clear, factual manner. Include images.
- Root Cause Analysis: Analyze the findings to determine the most probable root cause of the failure. Use tools like the Fishbone Diagram or "5 Whys".
- Conclusion & Recommendations: Summarize the root cause and provide specific, actionable corrective and preventive actions (CAPA).
- Identify Failure Modes: Brainstorm all the ways a process step could go wrong (e.g., "Furnace over-temperature").
- Identify Potential Effects: For each failure mode, what is the consequence? (e.g., "Parts are too brittle").
- Rate Severity (S): On a scale of 1-10, how serious is the effect? (1 = minor, 10 = catastrophic).
- Identify Potential Causes: What could cause this failure mode? (e.g., "Faulty thermocouple").
- Rate Occurrence (O): On a scale of 1-10, how likely is this cause to happen? (1 = very unlikely, 10 = almost certain).
- Identify Current Controls: What processes are in place to prevent or detect the failure? (e.g., "Quarterly SAT checks").
- Rate Detection (D): On a scale of 1-10, how likely are you to detect the failure before the product leaves? (1 = certain to detect, 10 = impossible to detect).
- Calculate Risk Priority Number (RPN): RPN = Severity × Occurrence × Detection.
- Take Action: Address the highest RPN items by implementing new controls to reduce Severity, Occurrence, or improve Detection.
General Calibration Steps
- Identify the instrument and the traceable standard to be used.
- Perform measurements on the standard according to a defined procedure (e.g., ASTM E18 for Rockwell).
- Record the "as found" readings from the instrument.
- Compare the readings to the standard's certified value and determine the error.
- If the error is outside the allowable tolerance, make adjustments to the instrument if possible.
- Record the "as left" readings after adjustment.
- Apply a calibration sticker showing the date performed, the due date for the next calibration, and the technician's initials.
Example: Calibrating a Rockwell Hardness Tester
Using certified hardness test blocks, perform a series of indentations across the block's surface. The average reading must be within the tolerance specified on the block's certificate (e.g., ±0.5 HRC). This should be done for blocks in the low, mid, and high ranges of the scale being used.
Example: Calibrating a Thermocouple
Place the test thermocouple and a calibrated reference thermocouple in a calibration furnace or block. Heat to a setpoint, allow to stabilize, and record the readings from both. The difference between the two readings is the error, which must be within the tolerance specified by standards like AMS 2750.
- Receiving Inspection: Verifying incoming raw material against certificates and specifications.
- In-Process Checks: Monitoring furnace parameters, atmosphere, and temperatures during the cycle.
- Final Inspection: Performing required tests (e.g., hardness, case depth) on finished parts to ensure they meet customer requirements.
- Calibration: Regularly calibrating all testing and measuring equipment (thermocouples, hardness testers, controllers).
- Certification: Generating test reports and certificates of compliance to accompany the finished product.
Key Industry Standards & Compliance
Adherence to industry standards is non-negotiable for quality assurance in heat treatment, especially for automotive and aerospace applications.
Hardness Conversion Tool
Enter a value in any field to see its approximate conversion in other common hardness scales. Conversions are based on ASTM E140 tables for steel.
• These conversions are approximate and intended for informational purposes only. They are based on standard charts for non-austenitic steels.
• Official conversions must be performed according to standards like ASTM E140, which involve detailed tables, not simple formulas.
• The accuracy of conversions can be affected by the material type, its microstructure, and the specific test method used.