Form hardening press hardening – quality control
Materials testing: problems and solutions
Tensile specimen production | Tensile test | Hardness test
Manufacture and test tensile specimens
How to achieve quality assurance
The mechanical strength and material homogeneity of car bodies are crucial for safety and energy efficiency. Therefore, body components are subjected to rigorous quality testing—particularly through tensile and hardness tests.
In safety-critical areas such as A- and B-pillars, crumple zones, and side impact protection, manufacturers rely on form- and press-hardened high-performance steels such as 22MnB5. Conventional steel would be too heavy here – up to 80 kg of additional weight per vehicle would increase fuel consumption.
However, taking suitable samples from these complexly shaped components and producing tensile specimens that comply with standards pose a major challenge – because the material is extremely hard, tough and difficult to machine.
During hot stamping, sheets are heated to 900 °C, formed, and shock-cooled, which changes the crystal structure. This increases the tensile strength to 1.400 to 1.600 MPa (tendency: 1.800 MPa) – while reducing weight.
Our specialized testing solutions ensure the quality of these innovative materials – for maximum safety and efficiency.
Precise samples for reliable quality assurance
To test the mechanical properties of high-strength 3D molded parts, tensile specimens in dumbbell or bone shapes are taken and subjected to rigorous tensile and hardness tests. This is the only way to reliably evaluate tensile strength, yield strength, and elongation and ensure correct curing. This quality assurance is essential for manufacturers and suppliers to maintain production standards.
Crucial for precise test results is not only the tensile testing machine, but above all the quality of the specimens. The challenge: The sampling points are often difficult to access, narrow, and short, while the specimens themselves, with lengths of only 100 to 165 mm, are extremely compact. Only with precisely taken, standard-compliant specimens can reliable measurement results be achieved.
Precise separation of the samples without distorting the material properties – a challenge
Conventional punching, as is common with softer materials, is not suitable for high-strength 3D parts, as it can generally only be punched with flat steel. In addition, standard forming sheets only have a tensile strength of 270–350 MPa. Hardened special steel, on the other hand, reaches 1.400–2.000 MPa. Since punching tools have only a slight increase in hardness at 2.300–2.400 MPa, the punch and die quickly become blunt and require frequent re-grinding, which drastically reduces tool life.
Alternative processes such as milling or water jet cutting also fail
Milling: Although carbide milling cutters are available, the form-hardened material is extremely tough – the chip cannot be removed cleanly, and the tool blunts within minutes of the first machining step. This leads to undesirable work hardening of the edges, making the material more brittle. During tensile testing, the material properties change, and the specimen can break in the wrong place, distorting the strain results.
Waterjet cutting: With this method, the required roughness of the machined edges of 6,3 Rz cannot be achieved, or only with great difficulty. The specimen may break prematurely and in the wrong place, which also leads to an incorrect, lower elongation.
The solution: laser cutting and grinding with the PSM2000
The only useful tool is the laser cutting. In most cases, the laser is a production step already used for trimming the 3D sheets after forming and hardening. It can also take on the task of taking a tensile test specimen from a flat area of the formed component. The machine operates quickly, flexibly, and fully automatically. However, this creates a heat-affected zone: At the cutting edge, the material is melted or heated to a depth of approximately 0,4 mm on each side, creating a heat-affected zone that changes the material properties. To ensure test results comply with standards, these changed areas must be removed through post-processing. This is the only way to ensure the yield strength (Rp0,2) remains unaltered and the required elongation is achieved.
The PSM2000 grinding machine offers the optimal solution for final processing, precisely contouring the tensile specimen and reliably removing the heat-affected zone of the laser cut – even with miniaturized tensile specimens. The unique PSM 2000 specimen grinding machines from SCHÜTZ+LICHT Prüftechnik GmbH are used worldwide – from Europe through North and South America to India and Mexico (VW, OPEL, FORD, MAGNA, COSMA, GESTAMP, SODECIA, VOESTALPINE, BENTELER, GEDIA, SNOP, TWB, BAO STEEL, LINDE&WIEMANN). They guarantee standard-compliant specimen preparation and prevent influences from thermal changes, uneven surfaces, or deviations in the gauge length.
Safe sample handling: The correct clamping
After machining, the next challenge follows: securely clamping the specimens for the tensile test. Due to their hardness and short length (shortest specimen: 100 mm, of which 32 mm is for the head area), conventional wedge grips quickly reach their limits. The initial clamping force is often insufficient, causing the specimens to simply slip under load. In addition, the clamping teeth quickly become blunt due to the hard material.
GALDABINI has developed a solution for this: the wedge screw grip, in which the wedge inserts are pressed into the wedge pocket of the grip via a screw spindle with high penetration force. This results in significantly improved grip – even with extremely hard materials. GALDABINI also offers special file inserts that securely hold even high-strength specimens up to 2.100 MPa.
Numerous well-known manufacturers such as FORD-Cologne, FORD-Saarlouis, FORD-Michigan and JMW Hövelhof rely on this innovative clamping technology to ensure reliable test results.
Wedge clamp + pyramid teeth | Wedge screw clamp + file inserts
Sample sizes and clamping force comparison
Due to the limited sampling options on 3D-formed body panels, smaller samples are usually used when testing form-hardened samples:
- ISO6892-1: 12,5×50 length 165 mm
and - ISO6892-1: not standardized: A30 sample length 100–130mm
************************************************** ****************** - ASTM A370: 12.5 wide (length 165 mm)
and - ASTM A370: 6 mm wide (length 100 mm)
************************************************** ****************** - The larger ISO 6892-1: 20 × 80 mm (length 250 mm) is only common for hardened samples in certain applications, particularly with suppliers of tailored blanks such as: BAO STEEL (Germany – Duisburg | Italy: Tito Scalo, San Gillio) | WORTHINGTON STEEL / BAO STEEL (TWB Monroe – Michigan | TWB Monterrey – Mexico)
Comparison of clamping forces
The clamping surface of the specimen head is crucial for the holding force in the tensile test. Here are the ratios in comparison:
| Sample type | head size | clamping surface | Relationship to the standard sample |
| Standard (20 × 80 mm, rare) | 30 × 50 mm | 1.500 mm² | 1,0 |
| Typical mold hardness test | |||
| Standard 12,5 × 50 mm | 20 × 45 mm | 900 mm² | ~1,7 times more penetration force |
| Mini tensile specimen ASTM 6 mm | 10 × 32 mm | 320 mm² | ~4,7 times more penetration force |
The smaller the clamping surface, the higher the force acting on the teeth.
Perfect combination for secure clamping
GALDABINI's innovative wedge-screw clamping technology ensures secure fixation of even the hardest and shortest tensile specimens.
✅ Maximum initial indentation force – the higher force per tooth presses the tips in – the wedge effect begins
✅ Ultra-sharp file inserts – specially hardened tips provide a secure “initial bite” and significantly improve clamping performance – even for sheets up to 2100 MPa
Why wedge-screw clamps are superior
🔹 Higher clamping force – In contrast to simple wedge clamps that are only moved by a spring, GALDABINI uses a spindle that presses the wedge jaws into the carrier with high force – similar to a vice.
🔹 Superior tooth geometry – While conventional clamping jaws only have milled pyramid teeth, we use extremely sharp, hardened file inserts for maximum grip.
Hydraulic solution for highest precision
For high test volumes, hydraulic grips ensure ergonomic operation and low wear. However, standard solutions have a weakness: Due to their design, conventional grip heads are only suitable for longer specimens.
Successful hot forming and hardening of the component
To ensure that the hot forming and hardening of the component was successful at all points, a hardness test is carried out. The Vickers hardness testing method is used. For this test, sheet metal strips are embedded, metallographically ground, and polished to demonstrate homogeneous hardening along their entire length. Vickers hardness test indentations are placed and measured at regular intervals. This process is very time-consuming and should ideally be carried out using an automated hardness testing machine. With this method, a virtual sample is roughly aligned under an overview camera in the overlay and then readjusted every 3 seconds. The setting, measurement, and evaluation of the Vickers HV10 hardness test indentations are then carried out fully automatically. This automation saves over 60 minutes of working time compared to manual testing. The results must be within the specified tolerances, as the tensile strength of the sample is directly correlated with the hardness. The hardness must be detectable along the entire length of the sample to ensure that the material is consistently hard and that quality is guaranteed.
SCHÜTZ+LICHT’s experience shows:
For tensile tests on small, form-hardened specimens from the automobile body, expensive hydraulic clamping devices are not necessarily required. A wedge-screw clamping fixture with hardened jaws also meets the requirements – and at a fraction of the cost.
For reliable quality assurance of form and press-hardened components, a comprehensive solution that covers all steps is recommended:
✔ Finishing of tensile specimens – laser sampling and finishing with the sample grinding machine PSM2000
✔ Tensile test – secure clamping and comfortable testing with a GALDABINI tensile testing machine
✔ Metallographic section – here too, preparation must be flawless METKON metallography preparation
✔ Automatic hardness testing – reproducible quality control with INNOVATEST Vickers fully automatic machines
Future-proof quality assurance in body construction
Hot stamping and press hardening are key processes in modern automotive manufacturing – from VW to BMW. They produce extremely hard, tough materials that can only be precisely cut using a laser. Subsequent grinding is essential for reproducible testing.
With the right clamping technology, even the smallest samples can be tested reliably. Furthermore, the automation of sample preparation and hardness testing offers significant savings potential – a clear competitive advantage for efficient quality assurance.
