1、 Classified by Material
1. Carbon steel needle
Mainly composed of low carbon steel or high carbon steel, it has high hardness and is easy to process and shape. The cost is affordable, but it is prone to rusting and usually requires surface nickel or chromium plating for rust prevention treatment. Commonly used in conventional industrial scenarios such as textile machinery and ordinary metal stamping.
2. Stainless steel needle
Mainly made of medical or industrial grade stainless steel materials such as 304 and 316L, it has excellent corrosion resistance and rust resistance. Commonly used in food processing equipment, medical devices, chemical pipelines, and coastal high salt spray environments.
3. Tungsten steel needle (hard alloy needle)
Made by sintering a mixture of tungsten carbide and cobalt, with a Mohs hardness of up to 8.5-9. It has extremely high wear resistance, high temperature resistance, and impact resistance, suitable for high-speed rotary machining and cutting and marking of high hardness materials, with a service life far exceeding that of ordinary steel needles

1. Heat treatment (quenching and tempering)
This is the process that determines the 'soul' of the steel needle. By heating the steel needle above the critical temperature and then rapidly cooling (quenching) it to make it extremely hard; Further low-temperature tempering is carried out to eliminate internal stress and achieve the optimal toughness balance. Steel needles that have not undergone proper heat treatment are either too soft and prone to bending, or too brittle and prone to breakage.
2. Surface treatment
Polishing: Remove grinding marks, make the needle body smooth like a mirror, and reduce friction during puncture.
Electroplating: nickel plating, chrome plating or gold plating, mainly for rust prevention and improving corrosion resistance, while also making the appearance of steel needles brighter.
Coating: High end industrial needles (such as sewing machine needles) are sometimes coated with coatings such as Teflon to reduce frictional heat during high-speed operation.
3. Needle hole processing (for needles with eyes)
If it is a sewing needle or injection needle, it is necessary to specifically punch or drill holes at the end of the needle and perform deburring treatment to ensure smooth threading without cutting the thread.

For industrial scenarios such as magnetic polishing, it is completely normal. The high temperature (50-100 ℃) generated by high-speed friction, combined with moisture and oxygen, will oxidize iron into black iron oxide, and the workpiece debris and oil stains will also wrap around the surface, making the needle turn black. This is not a quality issue.

Mild blackening (surface darkening): Soak in water and neutral detergent for 10 minutes, gently brush with a soft bristled brush, rinse thoroughly, then soak in anti rust solution and let it dry. It can still be used.
Severe blackening (black with lumps): Dissolve the oxide layer in 5% -8% dilute hydrochloric acid or oxalic acid for 20 minutes, rinse off, and soak in rust proof solution for 10 minutes. Pay attention to acid washing up to 2 times, as too many needles will make it soft and unable to move objects.

Slight bending (angle<30 °): Gently tap and correct on a flat hard surface, but the tensile strength of the corrected needle will decrease, and it is not recommended to use it in load-bearing scenarios.
Severe bending or folding: replace it directly and forcefully straighten it, which can easily cause breakage, splashing, and injury during use.