Surgical Steel vs Stainless Steel: What’s The Difference?
It's not uncommon to have questions about the distinctions between various steel types, especially if you're working in the machining and metalworking industries. Understanding the subtle, specific differences between various steel types is very important for business. After all, deciding which material is ideal for a certain application is a crucial stage in every project.
When metal materials are cut, bent, punched, or otherwise processed, their characteristics become more apparent, but knowing them in advance makes any machining job much easier. In this article, we'll discuss surgical stainless steel, what makes it different from typical stainless steel, and its different biomedical applications.
What is Surgical Steel?
People mostly mix stainless steel with surgical steel, but the truth is that one is a sub-group of the other. All surgical steel is stainless steel, but not all stainless steel is surgical steel. Unfortunately, surgical steel, sometimes referred to as surgical stainless steel or medical grade steel, has no formal definition. Surgical steel varieties are simply types of stainless steel with the highest levels of corrosion resistance.
As their name implies, these types of steel are mostly used in biomedical applications thanks to their hypoallergenic properties. These applications include but aren't limited to surgical instruments, body jewelry, orthopedic and dental implant devices, and many more uses.
Put simply; surgical steel is a typical low-carbon steel alloy (all steels are alloys) that contain high concentrations of chromium, which reacts with oxygen from the atmosphere to form a stable oxide bond that prevents staining and rust.
Stainless steel is also a non-porous, self-healing metal, which makes it much more hygienic for biomedical use. The former makes for easier sterilization of medical equipment, while the latter prevents any bacterial buildup in microscopic cracks within the material.
Implant-grade Stainless Steel
Surgical steels can also contain molybdenum, which further reduces its corrosion, and nickel, which reduces its brittleness. And this is where the trouble might start. Nickel is a reactive metal that can trigger an immune reaction within the patient's body. While it's rather suitable for making high-quality surgical equipment, it's very unstable for making surgical steel implants.
That's why surgical steel implants and bone fixtures are made with very low nickel content, or the steel alloy's chemical composition is entirely nickel-free, to avoid triggering an immune response within the patient's body.
In fact, it's highly recommended that the body piercing jewelry or surgical steel earrings contain no nickel, as it can trigger dermatologic reactions. It's mostly used in jewelry-grade gold alloys, but it's nowhere to be found in sterling silver.
Surgical Steel vs. Stainless Steel
As previously stated, not all stainless steel is surgical steel, and only those steels with the highest corroding resistance are intended for medical applications. Stainless steel is typically the most expensive type of steel, and conversely, surgical steel is typically the most expensive type of stainless steel.
Compared to "general use" steel like carbon steel and tool steel, stainless steels are more expensive and highly sought-after for use in cutlery, cookware, appliances, and kitchen fittings. Additionally, stainless steel is utilized in several construction applications as well as for a variety of components in the automobile and aerospace industries.
Biomedical applications are the only ones that can make use of surgical steel. Surgical steel is used to make various surgical tools, including forceps, retractors, and needle holders. Besides the lack of Nickel, stainless steels tend to have a higher concentration of Chromium — at least 13% compared to stainless steels standard concentration of 10.5%.
It's important to note that the concentration of Chromium can be higher — up to 30% — if you're manufacturing surgical non-magnetic elements. If you want to learn more about the specific differences between the two grades of stainless steel, refer to our Surgical Steel vs. Stainless Steel: What's the Difference guide.
Medical Grade & Surgical Stainless Steel
Here is a breakdown of the most common surgical-grade stainless steel that's typically used in healthcare:
SAE 304 Stainless Steel
304 is the most common grade of stainless steel, which contains anywhere between 18% and 20% chromium and between 8% and 10.5% nickel. 304 and 316 stainless steel are different from one another in that 316 has molybdenum, which improves corrosion resistance, whereas 304 does not. However, 304 stainless steel has a strong corrosion resistance, which is why 304 stainless steel tubing is widely used in the medical industry.
Due to its chemical inertness, this material is unaffected by sanitization or sterilizing procedures and doesn't react with any bodily tissue. Grade 304 stainless steel is strong and resilient and can withstand repeated use and abuse, making it perfect for paramedics, hospitals, and surgical applications.
SAE 316/316L Stainless Steel
Alloy 316/316L is a CrNiMo austenitic stainless steel designed to enhance the already improved corrosion resistance to Alloy 304/304L in mildly corrosive environments. While both are used in medical applications, 316L surgical steel has a lower carbon content, which makes it more resistant to corrosion.
SAE 420 Stainless Steel
This grade can't be used for implants. However, with a minimum chromium concentration of 12%, it possesses remarkable corrosion resistance in its annealed form and when polished, hardened, or surface grounded. 420 is resistant to air, food, fresh water, alkalis, and mild acids.
SAE 440 Stainless Steel
Medical grade steel 440 is sometimes called "razor-blade steel" and offers exceptional resistance to acids, alkalis, food, fresh water, and the atmosphere. In tempered, passivated, and hardened conditions, grade 440 also resists corrosion thanks to a smooth polished surface.
When annealed, grade 440 stainless steel is simple to process. It might be extremely challenging or even impossible to machine after hardening. The two grades of 440 that are employed in the medical sector are 440B and 440C. The amount of carbon in the two makes a difference. Both have a lot of carbon, but 440C has more of it, making this stainless steel challenging to manufacture.
SAE 630 Stainless Steel
Another substance unsuitable for implantable devices, but it's perfect for surgical equipment due to its great wear resistance. Due to the surgical steel's composition, which consists of roughly 17% chromium and 4% nickel, grade 630 is also known as 17-4.
Heat treatment can increase the strength and hardness of stainless steel 630 grade. Additionally, it features strong base metal and welds, reducing warpage and scaling. Its low carbon content gives it exceptional mechanical qualities, yet its corrosion resistance is comparable to that of 304.
Medical metals: common applications for stainless steels
| Grade | 304 | 316 | 316L | 420 | 440B | 440C | 630 (17- 4) |
|---|---|---|---|---|---|---|---|
| Orthopedic implant devices | X | X | |||||
| Dental implant devices | X | ||||||
| Surgical and dental instruments | X | X | X | X | X | X | |
| Pharmaceutical equipment: sample & material handling | X | ||||||
| Stainless steel tubing | X | ||||||
| Bone fixation | X | ||||||
| Containers for hazardous materials | X | ||||||
| Wires | X | ||||||
| Curettes | X | ||||||
| Plates and screws | X | ||||||
| Prostheses | X | ||||||
| Syringes and needles | X | ||||||
| Catheters | X | ||||||
| Sensor probes | X | ||||||
| Hospital furniture | X | X |
Summary
Lacking any official definition, we can conclude that only the stainless steels with the highest levels of corrosion and the lowest chances of inducing an immune response can be used as surgical stainless steels. If you want to learn more about stainless steel materials, visit Reid Supply and refer to their Engineer Resource Guides.