Austenite Testing in Steel

Austenite inspection in steel mainly conducts scientific testing and analysis on the retained austenite content determination, austenite grain size rating, austenitizing state evaluation, austenite microstructure morphology observation and austenite stability analysis, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Retained austenite content determination: The retained austenite refers to the austenite phase that has not transformed and is retained to the room temperature during the martensitic transformation of the steel. Its content is the key indicator for measuring the quality of the heat treatment process, controlling the dimensional stability of the parts and predicting the fatigue life. Both the excessively high and low contents will have the significant influence on the comprehensive mechanical properties of the material.

Austenite grain size rating: The grain size reflects the average size of the austenite grains under the specific heating conditions, and is the important basis for rating the inherent grain size, overheating degree of the steel and predicting the mechanical properties after the heat treatment (such as the strength and toughness). It is usually rated in accordance with the standards such as GB/T 6394 or ASTM E112.

Austenitizing state evaluation: Evaluates whether the steel reaches the complete austenitization during the heat treatment process, as well as the uniformity of the austenite composition. This testing has the decisive influence on the structure and performance of the subsequent cooling transformation products, and is the core link of the process verification.

Austenite microstructure morphology observation: The grain boundary morphology, twin characteristics of the austenite and whether the non-metallic inclusions exist are observed through a metallographic microscope or scanning electron microscope. The morphology analysis helps to trace the processing history and heat treatment defects of the material.

Austenite stability analysis: Evaluates the tendency of the austenite to undergo the phase transformation under the stress or in the subsequent cooling process. For the high-carbon steels, bearing steels and some TRIP/TWIP steels, the austenite stability directly affects their service performance, and needs to be evaluated through the specific thermal simulation or mechanical testing.

Testing Scope

Structural steels and tool steels: Including the carburizing steels, quenched and tempered steels, spring steels and die steels. The retained austenite content after quenching needs to be detected, to optimize the tempering process, control the deformation and ensure the hardness and wear resistance, such as the key components like the gears and bearing rings.

Stainless steels: Especially for the austenitic stainless steels (such as the 304 and 316 series) and duplex stainless steels. The testing emphasis is on confirming the austenite phase ratio (such as the phase ratio control of the duplex steels), evaluating the intergranular corrosion susceptibility and the content of the deformation-induced martensite after the cold working.

Advanced high-strength steels (AHSS): Such as the TRIP steels (transformation-induced plasticity steels) and Q&P steels (quenching and partitioning steels). Their excellent performance depends on the metastable austenite retained in the matrix, and the precise detection of its content, distribution and stability is the core of the material research and development and quality control.

Cast steels and welded heat-affected zones: The casting and welding processes will produce the unique austenite formation and transformation conditions. The detection of the austenite characteristics in this region is crucial for evaluating the structure uniformity, eliminating the casting defects and predicting the performance of the welded joints.

Failure analysis samples: For the parts that fail prematurely (such as the fatigue fracture and wear failure), the austenite state at the key parts is detected, which can trace whether the failure is related to the improper heat treatment, abnormal materials or design defects.

Testing Methods

X-ray diffraction method: This is the most classic and precise quantitative method for determining the retained austenite content. Its principle is to calculate by utilizing the intensity difference of the diffraction peaks of the austenite and ferrite/martensite. This method is non-destructive with the accurate results, and is the preferred method in the laboratories and scientific research.

Metallographic method: The austenite grain boundaries are displayed or the different phases present the different colours through the specific etchants (such as the heat tinting, electrolytic etching or Beraha reagent), and the qualitative or semi-quantitative analysis is carried out under an optical microscope. This method is intuitive, and is commonly used for the grain size rating and structure morphology observation.

Magnetic method: Based on the principle that the austenite is paramagnetic while the ferrite/martensite is ferromagnetic. By measuring the saturation magnetization or magnetic permeability of the material and comparing it with the standard samples, the austenite content is calculated. This method is fast and portable, and is suitable for the rapid screening of the field and large batches of products.

Electron backscatter diffraction: The EBSD technology can carry out the crystal orientation and phase analysis of the micro-areas on the polished sample surface, and can precisely characterize the distribution, grain size, orientation of the austenite phase and the orientation relationship with the adjacent phases. It has the high spatial resolution and is a powerful tool for studying the fine multi-phase microstructures.

Thermal analysis method: Such as the dilatometry. By precisely measuring the dimension changes of the sample during the heating or cooling process, the start and end temperatures of the austenite transformation (Ac1, Ac3, Ms, etc.) are determined, used to study the austenitizing kinetics and the phase transformation process.

Testing Standards

Domestic Standards:

1. GB/T 6394-2017, Determination of estimating the average grain size of metal

2. GB/T 228.1-2021, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

3. GB/T 229-2020, Metallic materials — Charpy pendulum impact test method

4. GB/T 232-2010, Metallic materials — Bend test method

5. GB/T 230.1-2018, Metallic materials — Rockwell hardness test — Part 1: Test method

International Standards:

1. ASTM E112, Standard test methods for determining average grain size

2. ASTM E975, Standard practice for X-ray determination of retained austenite in steel with near random crystallographic orientation

3. ISO 6892-1, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

4. ISO 9227, Corrosion tests in artificial atmospheres — Salt spray tests

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

X-ray diffractometer: The core equipment for the retained austenite quantitative analysis. The modern XRD is equipped with the high-temperature accessories, which can carry out the in-situ phase transformation research. During the use, attention needs to be paid to the sample preparation, diffraction peak separation and fitting, and the selection of the appropriate standard methods (such as ASTM E975).

Optical metallographic microscope and image analysis system: Used for the austenite grain size rating and structure observation. The system needs to be equipped with the professional metallographic sample preparation equipment (cutting, mounting, grinding and polishing) and computer image analysis software, to realize the automatic identification and statistics of the grains.

Field emission scanning electron microscope: The FESEM equipped with the EBSD probe is the top-level equipment for the submicron austenite structure analysis. It can provide the extremely high spatial resolution and accurate phase identification results, and is particularly suitable for studying the nanoscale retained austenite in the advanced high-strength steels.

Magnetic austenite measuring instrument: A dedicated portable or bench-type instrument, which quickly measures the austenite content based on the magnetic principle. The equipment is usually calibrated with the standard samples, and is simple to operate with the fast readings, widely used in the quality control at the production sites.

Dilatometer: Used to determine the transformation points and study the kinetics of the austenite formation and decomposition. This equipment can carry out the programmed heating/cooling of the sample under the controlled atmosphere, and precisely record its length changes. It is the basic experimental equipment for formulating the heat treatment processes.