Hot-work Die Steel Plate Testing

Hot-work die steel plate inspection mainly conducts scientific testing and analysis on the chemical composition analysis, macroscopic and microscopic structure examination, hardness testing, mechanical property testing, impact toughness testing and thermal fatigue property testing, 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

Chemical composition analysis: Detects the contents of the alloying elements such as the carbon (C), chromium (Cr), molybdenum (Mo), vanadium (V) and tungsten (W) in the hot-work die steel. The precise chemical composition is the basis for ensuring that the material has the excellent high-temperature strength, thermal stability and thermal fatigue resistance, directly affecting the service life and performance of the die.

Macroscopic and microscopic structure examination: The metallurgical defects such as the segregation, porosity and cracks of the ingots or steels are observed through the low-magnification examination; the microstructures of the annealed state and quenched and tempered state, such as the morphologies, distributions and grain sizes of the pearlite, bainite and martensite, are analysed with a metallographic microscope, evaluating the uniformity and stability of the structures.

Hardness testing: Measures the hardness values of the material at the room temperature and high temperatures, including the Brinell hardness (HB), Rockwell hardness (HRC) and Vickers hardness (HV). The hardness is the key indicator for measuring the strength, wear resistance and plastic deformation resistance of the die steel, and the hardness retention after the high-temperature tempering especially needs to be focused on.

Mechanical property testing: Mainly including the room-temperature and high-temperature tensile testing, determining the tensile strength, yield strength, elongation after fracture and reduction of area of the material. The high-temperature performance testing is the core of the hot-work die steel, used to evaluate its load-bearing capacity and deformation resistance at the working temperatures.

Impact toughness testing: The Charpy V-notch impact testing is adopted to determine the impact absorbed energy of the material at the different temperatures. The excellent impact toughness is the key guarantee for preventing the early brittle fracture or cracking of the die under the rapid heating and cooling or impact loads.

Thermal fatigue property testing: Simulates the cold and hot cycle conditions of the die during the actual work. By inducing the thermal stress cycles, the tendency of the surface cracking (thermal cracks) and the crack growth rate of the material are evaluated. This is one of the most direct and important performance indicators for evaluating the service life of the hot-work die steel.

Hardenability testing: The hardenability curve of the steel is determined through the end quenching test (Jominy test), understanding the hardness distribution along the longitudinal direction at the different cooling rates. The good hardenability ensures that the core of the large-cross-section die can also obtain the required structures and properties.

Testing Scope

Incoming acceptance testing of the raw materials: The incoming inspection of the procured die steel billets, bars and plates is carried out, ensuring that their chemical compositions, low-magnification structures and delivery state hardness meet the requirements of the procurement technical agreements, controlling the quality from the source.

Testing of the forged and rolled billets and semi-finished products: The blanks after the forging and rolling are tested, emphatically focusing on their flow line distributions, whether there are the forging cracks, the structure refinement degrees and the hardness uniformity after the stress-relief annealing, preparing for the subsequent processing.

Heat treatment process and finished product testing: The comprehensive performance testing of the dies after the final heat treatments such as the quenching and tempering is carried out, including the hardness, metallographic structures and deformation amounts, ensuring that they achieve the designed comprehensive mechanical properties and service states.

Failure analysis testing: For the dies that have the failures such as the early cracking, collapse, wear and thermal fatigue cracking during the use, the systematic testing and analysis are carried out, and the root causes of the failures are found through the means such as the fracture analysis, composition comparison and structure observation.

Testing of the different steel types: The testing scope covers the mainstream hot-work die steel types, such as the high-toughness 3Cr2W8V and 5CrNiMo, the high-thermal-strength H13 (4Cr5MoSiV1) and H11, as well as the copper alloy die materials with the high thermal conductivity. The testing items have the different emphases according to their characteristics.

Testing of the surface modified layers: For the dies after the surface strengthening treatments such as the nitriding, PVD/CVD coating and laser cladding, the thickness, hardness gradients, bonding strength, phase compositions and surface residual stresses of the modified layers are detected, evaluating the strengthening effects.

Testing Methods

Spectrometry analysis method: The spark direct-reading optical emission spectrometer or inductively coupled plasma optical emission spectrometer is adopted for the fast quantitative analysis of the chemical composition. This method has the high precision and fast speed, and is the routine means of the analysis before the furnace and the material acceptance.

Metallographic examination method: In accordance with the standards such as GB/T 13298, the specimens are cut, mounted, ground, polished and etched, and the microstructures, grain sizes, non-metallic inclusion grades and decarburization layer depths are observed and rated under a metallographic microscope or scanning electron microscope.

Hardness testing method: In accordance with the standards such as GB/T 230.1 (Rockwell), GB/T 231.1 (Brinell) and GB/T 4340.1 (Vickers), the corresponding hardness testers are used for the testing. The high-temperature hardness testing needs to be carried out in the vacuum or protective atmosphere hardness testers equipped with the heating devices.

Tensile and impact testing method: The room-temperature tensile testing is based on GB/T 228.1, and the high-temperature tensile testing is based on GB/T 4338, completed on a universal material testing machine. The impact testing is based on GB/T 229, using a pendulum impact testing machine at the specified temperatures.

Thermal fatigue testing method: The self-restraint or external restraint testing methods are usually adopted. One end or the whole of the specimen is subjected to the programmed heating-cooling cycles. After a certain number of cycles, the numbers, lengths and depths of the surface cracks are observed and counted through a stereomicroscope, evaluating the thermal fatigue resistance of the material.

Non-destructive testing method: Including the ultrasonic flaw detection (UT) for detecting the internal metallurgical defects and forging cracks; the magnetic particle flaw detection (MT) or penetrant flaw detection (PT) for detecting the fine cracks on the surfaces of the material or dies. These methods evaluate the quality without damaging the workpieces.

End quenching testing method for the hardenability: In accordance with the standard GB/T 225, after the standard specimen is heated to the austenitizing temperature, one end is cooled by the water spraying, and then the hardness distribution is measured along the length direction of the specimen, drawing the hardenability curve.

Testing Standards

Domestic Standards:

1. GB/T 13298, Inspection methods of microstructure for metals

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

3. GB/T 4340.1-2009, Metallic materials — Vickers hardness test — Part 1: Test method

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

5. GB/T 4338, Metallic materials — Tensile testing at elevated temperature

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

7. GB/T 225, Steel — Hardenability test by end quenching (Jominy test)

International Standards:

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

2. 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

Direct-reading optical emission spectrometer: Used for the fast and accurate determination of the contents of all the main alloying elements and trace residual elements in the die steel. The instrument needs to be regularly calibrated with the standard samples, to ensure the accuracy and reliability of the analysis results.

Metallographic microscope and image analysis system: Including the optical metallographic microscopes and scanning electron microscopes (SEM). The former is used for the routine structure observation and rating; the latter, combined with the energy dispersive spectrometer (EDS), can carry out the high-magnification structure morphology observation and micro-area composition analysis, and is the core equipment of the failure analysis.

Universal material testing machine: Equipped with the high-temperature furnaces, extensometers and control systems, it can complete the mechanical property tests such as the tensile, compression and bending from the room temperature to 800 ℃ or even higher temperatures, precisely measuring the stress-strain curves of the material at the high temperatures.

Hardness tester series: Including the bench-type Rockwell/Brinell/Vickers hardness testers, portable Leeb hardness testers and the dedicated high-temperature vacuum hardness testers. The Vickers hardness tester, because of its wide test force range, is particularly suitable for detecting the micro-hardness gradients of the surface modified layers.

Impact testing machine and low-temperature bath: The pendulum impact testing machine is used to determine the impact toughness of the material. The supporting low-temperature baths or high-temperature furnaces can provide the required test environment temperatures for the specimens, to evaluate the toughness transition behaviour of the material at the different temperatures.

Thermal fatigue testing machine: A dedicated equipment, usually including a high-frequency induction heating system, controllable quenching medium spraying system, specimen fixtures and cycle counting control system, which can precisely simulate the cold and hot alternating working conditions of the die.

Non-destructive testing equipment: The ultrasonic flaw detector, combined with the probes of the different frequencies and angles, is used to detect the internal defects; the magnetic particle flaw detector or penetrant testing reagent kits are used to detect the discontinuous defects on the surfaces and near-surfaces of the workpieces. They are the important means of the in-service inspection of the dies.