Stainless Steel Knife Testing

Stainless steel cutlery and knife inspection mainly conducts scientific testing and analysis on the material composition analysis, hardness testing, corrosion resistance testing, metallographic structure examination, sharpness and edge retention testing and surface quality and coating 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

Material composition analysis: Through the means such as the spectral analysis, the contents of the main alloying elements in the knife stainless steel, such as the chromium, nickel, molybdenum, carbon and vanadium, are precisely determined. This is the basis of judging whether it belongs to the specific designations (such as 304, 316, 440C and 9Cr18MoV), ensuring that the material meets the corrosion resistance, hardness and toughness requirements needed by the design.

Hardness testing: This is the core indicator of measuring the edge retention and wear resistance of the knives. Mainly measured with the Rockwell hardness testers (HRC). For the high-quality knives, the edge hardnesses are usually required to reach 55-62 HRC. The testing needs to be carried out at the multiple points on the effective parts of the cutting edges, to ensure the uniformity of the hardness.

Corrosion resistance testing: Evaluates the ability of the stainless steel to resist the rust. The common methods include the salt spray testing (such as the neutral salt spray testing NSS), placing the knives in the salt spray environments of the specific concentrations and observing whether the defects such as the rust spots and corrosion pits appear on the surfaces within the specified times, to verify their chromium contents and surface passivation film qualities.

Metallographic structure examination: Through the specimen preparation, etching and observation under the metallographic microscopes, the micro-structure of the stainless steel, such as the morphologies, distributions and grain sizes of the martensite and austenite, is analysed. This testing can reveal whether the heat treatment processes of the material are proper, and whether there are the internal defects such as the overheating, overburning or uneven structures.

Sharpness and edge retention testing: The sharpness is usually evaluated through the smoothness of cutting the specific media (such as the paper and hemp ropes). The edge retention is evaluated by simulating the use, such as carrying out the multiple cutting under the specified materials and conditions and then testing the attenuation degree of the sharpness again, comprehensively evaluating the micro toughness and wear resistance of the cutting edges.

Surface quality and coating testing: Checks whether there are the defects such as the cracks, scratches, pits and burrs on the cutting edge and blade surfaces. For the knives with the coatings (such as the DLC coatings and titanium coatings), the coating thicknesses, adhesion (cross-cut testing) and uniformity also need to be tested, ensuring their aesthetics and functionality.

Testing Scope

Kitchen knives: Covering the Chinese kitchen knives, Western chef's knives, fruit knives and bone chopping knives, etc. The testing emphases are the compliance of the food contact materials (such as the heavy metal migration amounts), the resistance to the food acid and alkali corrosion, the sharpness of the cutting edges and the anti-chipping properties during the long-term use.

Outdoor and tactical knives: Including the hunting knives, survival knives and multi-function tool knives, etc. The testing requirements are more stringent. In addition to the routine items, the overall strengths of the blades (bending resistance and impact resistance testing), the firmness of the handle connections, and the corrosion resistance and reliability under the harsh environments are particularly emphasized.

Professional industrial knives: Such as the turning tools and milling inserts for the machining, and the slicing knives for the food industry, etc. The testing focuses on the red hardness, wear resistance and dimension stability under the high-speed, high-temperature and high-load working conditions. The materials mostly adopt the high-performance high-speed steels or cemented carbides.

Beauty, hairdressing and medical instrument knives: Such as the scissors and surgical blades. The testing needs to follow the strict hygiene and safety standards, emphasizing the biocompatibility of the material, the extremely high corrosion resistance (can withstand the repeated sterilization), and the ultimate precision and smoothness of the cutting edges, avoiding the tissue adhesion or damage.

Knife raw materials and semi-finished products: Referring to the stainless steel plates and bars used for manufacturing the knives, as well as the knife blanks after the forging and rolling. The testing focuses on the original compositions of the material, metallurgical qualities (non-metallic inclusion grades), internal cracks (ultrasonic flaw detection) and initial hardnesses, controlling the quality from the sources.

Surface treatment process verification: The testing is carried out on the knives after the surface treatments such as the polishing, stone washing, sand blasting, etching and coating. The surface roughness, glossiness, anti-fingerprint properties after the treatments and whether the treatment processes introduce the new stresses or micro-cracks are evaluated.

Testing Methods

Spark optical emission spectrometry (OES) and X-ray fluorescence spectrometry (XRF): Used for the fast and non-destructive material composition analysis. The OES has the high precision and is suitable for the precise identification in the furnace front and laboratories; the XRF is portable and can be used for the fast on-site screening and designation differentiation. The combination of the two ensures the accuracy and efficiency of the composition control.

Rockwell hardness testing method (HRC): The most commonly used cutting edge hardness testing method. The diamond cone indenters are pressed into the specimen surfaces under the preliminary loads and main loads, and the hardness values are converted by measuring the indentation depth differences. The method is simple and fast, has the moderate requirements for the specimen surfaces, and can directly reflect the heat treatment effects.

Salt spray testing method: According to the standards such as GB/T 10125 and ISO 9227, the knife specimens are placed in the closed salt spray test chambers, continuously sprayed with the sodium chloride solution of a certain concentration, simulating the marine or industrial atmospheric environments. The corrosion resistance grades are quantitatively evaluated by observing the times (hours) of the beginning of the rust.

Metallographic specimen preparation and observation method: Including a series of steps such as the sampling, mounting, grinding, polishing and chemical or electrolytic etching, and finally observing, photographing and analysing the micro-structures under the metallographic microscopes or scanning electron microscopes. This is the indispensable micro analysis method of studying the material essence and diagnosing the process problems.

Cutting performance simulation testing method: Through the standardized test benches, the knives are made to repeatedly cut the test pieces of the specific materials (such as the paper with the silicone inks and standard hemp ropes) at the fixed angles, pressures and strokes, recording the numbers of the effective cuts or measuring the cutting force changes, scientifically evaluating the edge retention.

Adhesion cross-cut testing method: Used to evaluate the bonding strengths between the coatings and the knife body materials. The dedicated cross-cut testers are used to scribe the grids on the coating surfaces, the dedicated tapes are pasted and quickly torn off, and the adhesion grades are rated according to the coating falling off proportions in the grid areas (usually grade 0 is the best and grade 5 is the worst).

Testing Standards

Domestic Standards:

1. GB/T 3280, Cold rolled stainless steel plate, sheet and strip

2. GB/T 11170, Stainless steel — Determination of multi-element contents — Spark discharge atomic emission spectrometric method (routine method)

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

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

5. GB/T 10125-2021, Corrosion tests in artificial atmospheres — Salt spray tests

International Standards:

1. ISO 6508-1, Metallic materials — Rockwell hardness test — Part 1: Test method

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 spectrometer: The core equipment of the composition analysis. The samples are excited through the arc or spark to make the atoms emit light, and the characteristic spectra are obtained through the dispersion systems and their intensities are detected, thereby calculating the contents of the various elements. It has the advantages of the fast analysis speed, high precision and simultaneous determination of the multiple elements, and is the necessary instrument for the steel plants and large knife manufacturers.

Rockwell/Vickers hardness tester: The standard equipment of the hardness testing. The Rockwell hardness testers are used for the routine cutting edge hardness testing; the Vickers hardness testers (HV) are suitable for the hardness gradient testing of the smaller and thinner areas or the surface hardened layers such as the nitrided layers. Their indentations are square, and the measurement is more precise.

Salt spray test chamber: The dedicated environmental simulation equipment for the corrosion resistance testing. The chambers are made of the corrosion-resistant materials and equipped with the spraying systems, heating devices, saturation barrels and precise temperature and humidity control systems, which can create and maintain the stable and repeatable salt spray corrosion environments.

Metallographic microscope and image analysis system: The key equipment of the metallographic structure observation and analysis. The microscopes provide the high-magnification micro images, and the digital image analysis systems can carry out the automatic or semi-automatic quantitative measurement and statistics of the grain sizes, phase proportions and inclusion sizes, making the analysis results more objective and data-based.

Knife sharpness tester: The professional property testing equipment. Usually integrating the force sensors, motion control modules and data acquisition systems, it can precisely control the cutting speeds, angles and depths, record the resistance changes during the cutting in real time, and replace the subjective feel evaluation with the objective data.

Coating thickness gauge and adhesion tester: The coating thickness gauges often adopt the magnetic method (measuring the non-magnetic coatings on the iron and steel substrates) or eddy current method, which can quickly measure the coating thicknesses non-destructively. The adhesion testers include the cross-cut testers, tapes and possibly the equipped pull-off adhesion testers, used to quantitatively evaluate the coating bonding strengths.