From the testing laboratory: damage analysis of a kitchen knife
4. April 2023
Illustrative image of a kitchen knife.
A kitchen knife from the staff break room breaks while cutting an apple – a seemingly trivial incident. However, the subsequent materials engineering investigation reveals a classic example of intergranular corrosion in a martensitic chromium steel. The investigation aims to identify the cause of the damage and recommend remedial measures in accordance with VDI Guideline 3822.
1. Description of the damage
The object of the investigation is an ordinary kitchen knife from our staff break room. With energetic assistance from our sales department, the blade broke while cutting a perfectly ordinary Malus domestica (apple) – fortunately without injuring the employee.
2. Condition
The knife, dulled by many years of washing and cutting, shows wear consistent with its age (Fig. 3). No major plastic deformation or pronounced corrosion damage is visible. Closer examination of the fracture surface, however, reveals signs of corrosion on the half facing the cutting edge (Fig. 4).
3. Instrumental examinations
Chemical analyses by optical emission spectrometry
Results of Vickers hardness testing
Note: The requirements for the chemical composition of material 1.4021 (X20Cr13) in accordance with DIN EN 10088-2:2014-12 are met. The measured hardness is also within the usual range for this material.
4. Fractography using a stereomicroscope
Before the fractographic examination, the fracture surface was cleaned in an ultrasonic bath using a non-corrosive fracture-surface cleaner (Figs. 5 and 6).
After cleaning, three fracture zones with different colours and morphologies can be distinguished:
- Rubbed or hammered area: This area was produced by repeated contact between the fracture surfaces and represents the oldest part of the fracture.
- Crystalline, corroded area: This area must already have been open before final fracture, as otherwise no signs of corrosion could have formed.
- Crystalline, shiny area: This is the freshly formed final overload fracture produced when the employee broke the knife.
5. Fractography using a scanning electron microscope (SEM)
Examination of fracture zones 1 and 2 using a scanning electron microscope reveals a predominantly intergranular fracture surface with widened, gaping grain boundaries (Fig. 7). At higher magnification, local dimple structures are visible, indicating local plastic deformation. The delayed progression of the fracture and the observed fracture-surface morphology together point to a corrosion-related fracture mechanism.
6. Examination of a metallographic section using a light microscope
Examination of transverse section M1 (see the image detail for its position) reveals several areas of pitting corrosion (Fig. 9). Severe corrosion along the prior austenite grain boundaries is visible in their immediate vicinity (Fig. 10). This form of selective corrosion is known as intergranular corrosion. Etching reveals a hardened and tempered martensitic microstructure.
7. Conclusions from the damage analysis
In this case, the fracture resulted from intergranular corrosion. This form of corrosion attacks certain areas of the microstructure more severely than others. In stainless steels such as the martensitic chromium steel X20Cr13 used here, the prior austenite grain boundaries are particularly susceptible.
The reasons for this are the lower corrosion resistance of grain boundaries as microstructural discontinuities and the possible formation of chromium carbides at the grain boundaries. These carbides bind chromium from the surrounding matrix, leaving less chromium available to form the protective chromium oxide layer.
The measured chromium content of approximately 12.5% is only just above the minimum content of 10.5% required to form a passive layer. Natural segregation, local chromium depletion and the structural weakness of grain boundaries may therefore already be sufficient to initiate intergranular attack.
8. Cause of the damage and remedial measures
The most likely cause of the damage is a failure to observe a well-known rule of kitchen practice: sharp kitchen knives do not belong in the dishwasher. A subsequent bite test of the Malus domestica involved in the incident certainly revealed no unacceptably high hardness values in this otherwise unremarkable break-time snack.
The recommended remedial measures are therefore a yellow washing-up sponge, lukewarm water and a slightly less vigorous approach by our sales department.
