Description

AISI 440C (1.4125, S44004) Stainless Steel

Stainless Steel – Grade 440 (UNS S44000)

Introduction

What is 440C?

A martensitic stainless steel. It’s composed of these major alloy elements (I don’t list the minor elements since they don’t play a major role) :

• Carbon: 0.95-1.25% This increases hardenability and wear resistance, but decreases toughness in higher amounts.
• Manganese: .45% This is added to reduce brittleness and improve forgeability, hardenability, and reducing deformation.
• Silicon: .30% This is a deoxidizer and improves hot-forming properties.
• Chromium: 17.00-18.00% Added only to high alloy tool steels, this improves hardenability, high wear resistance, toughness, and corrosion resistance.
• Molybdenum: .50% Improves deep hardening and toughness.

These basic elements (along with iron) are a simple combination that works well together, and has for many decades.

This alloy is technically classified as UNS (Unified Numbering System) S44044. It’s also classified as SAE (Society for Automotive Engineers) as 51440C. The American Iron and Steel Institute classifies it as 440C martensitic standard stainless steel. Most modern knifemakers simply call it 440C.

440A: this is a hardenable stainless steel alloy, hardenable to a higher hardness than 420 series stainless steels (which are only hardenable to 52 HRC!). 440A has good corrosion resistance, and is used in less expensive bearings and in harder surgical tools (rare). It has 0.6 to 0.75 percent carbon, about the same carbon content as most steel springs. Its advantage is that is cheaper than 440B and slightly more corrosion resistant.

440B: this is a hardenable stainless steel alloy, hardenable to a higher hardness than 440A. It has good corrosion resistance and is used in cutlery (economy), valves, and instrument bearings, where high wear is less important than high corrosion resistance. It has 0.75 to 0.95 percent carbon.

440C: this is also a hardenable stainless steel alloy, hardenable to a higher hardness than 440B. It has good corrosion resistance, and is used in ball bearing balls and races, high pressure nozzles, valve seats and high wear components. It has 0.95 to 1.20 percent carbon. As detailed in the Machinery’s Handbook: “This steel has the greatest quenched hardness and wear resistance upon heat treatment of any corrosion-resistant or heat-resistant steel.”

All three of these steels have the same amount of chromium, from 16 to 18 percent. They are all high chromium martensitic standard stainless steels. All of their other alloy elements are about the same, including manganese, silicon, phosphorus, sulfur, and molybdenum. Truly, the difference in these three is the carbon content, which is substantial. The higher carbon content in 440C yields a much more wear resistant knife blade.

It’s important to understand the background of this special type of steel. Stainless steels were invented roughly around the beginning of the 20th century. Many different alloys were experimented with, and it didn’t take long to discover that chromium, the hardest of all known metals on the Periodic Table of Elements, was highly beneficial as an alloy element to steel. It was discovered that the addition of this extremely corrosion resistant, hard, brittle, and lustrous metal would make steel (basically iron with a tiny amount of carbon) a lot harder, a lot more corrosion resistant, and stronger overall. Another curious fact surfaced too, that the higher the carbon content in the chromium steel, the less corrosion resistant the stainless steel becomes. More carbon makes the steels more hardenable, and thus more wear resistant, but decreases the corrosion resistance. The issue is one of balance, a term frequently referred to in machinists’ and engineers’ references. This balance depends on the use, exposure, strength, and corrosion resistance needed for particular applications.

Grade 440C stainless steels are high carbon steels, which attain the highest hardness, wear resistance and strength of all stainless steel grades after heat treatment. These properties make this grade suitable for applications such as valve components and ball bearings. Grade 440A and 440B stainless steels, on the other hand, have similar properties – except for a slightly lower percentage of carbon in grade 440A.

These grades have a corrosion resistance lower than that of other austenitic grades. The applications of martensitic steels are limited by the loss of strength caused by over-tempering at high temperatures, and loss of ductility at temperatures below zero.

All three forms of grade 440 steels are commonly used. However, grade 440C is more readily available than the other standard grades. Grade 440F, a free-machining type of grade 440 series, is also available with a high carbon content similar to that of grade 440C. Martensitic steel grades are high-hardness steels, usually fabricated using techniques that require hardening and tempering treatments.

Specifications

The following specifications cover Stainless Steel 440 C

• AISI 440C
• AMS 5618
• AMS 5630
• AMS 5880
• ASTM A276
• ASTM A314
• ASTM A473
• ASTM A493
• ASTM A580
• DIN 1.4125
• QQ S763
• UNS S44004

Key Properties

The following section discusses the properties of grade 440 bar products covered under ASTM A276. These values may not be similar to that of other forms such as forgings and plates.

Composition

The chemical compositions of various elements of grade 440 stainless steels are tabulated below:

Table 1. Chemical composition ranges of grade 440 stainless steels

Mechanical Properties

Table 2. Mechanical properties of grade 440C stainless steels

* Annealed properties are typical for Condition A of ASTM A276# Brinell hardness is ASTM A276 specified maximum for annealed 440A, B and C.

Physical Properties

The following table outlines the physical properties of grade 440 stainless steels:

Table 3. Typical physical properties of grade 440 stainless steels

Grade Specification Comparison

Grade specifications for 440 stainless steels are given in the following table:

Table 4. Grade specifications of grade 440 stainless steels

Possible Alternative Grades

Suitable alternatives to grade 440 stainless steels are listed in the table below:

Table 5. Possible alternative grades to 440 stainless steels

Corrosion Resistance

Grade 440 stainless steels exhibit excellent resistance to mild acids, alkalis, foods, fresh water and air. A smooth polished surface also helps grade 440 steels resist corrosion in tempered, passivated and hardened conditions.

For optimum corrosion resistance surfaces must be free of scale and foreign particles.
Finished parts should be passivated.

Grade 440C steels exhibit corrosion resistance similar to that of grade 304 steels.

440C has a corrosion resistance somewhat similar to 410 grade, but lower than 431 grade, also lower than most of the 400 series ferritic stainless steels and all of the 300 series austenitic stainless steels.NB. It has optimum corrosion resistance in the hardened and tempered condition when tempered below 400^oC. Hardening from 1090^oC will ensure better carbide solution, and therefore better corrosion resistance, but minimum soaking time should be allowed at this temperature otherwise excessive grain growth can occur. Polishing will further develop its corrosion resistance. It is not recommended for use in the annealed condition.It is most important that oxygen is always allowed to circulate freely on all stainless steel surfaces to ensure that a chrome oxide film is always present to protect it. If this is not the case, rusting will occur as with other types of non stainless steels.

Principal Design Features : This is a high carbon martensitic stainless with moderate corrosion resistance good strength and the ability to obtain and keep excellent hardness (Rc 60) and wear resistance

Heat Resistance

Grade 440 stainless steels should not be used at temperatures below the relevant tempering temperature, due to loss of mechanical properties caused by over-tempering.

Heat Treatment

Annealing — Grade 440 stainless steels are full annealed at 850 to 900°C, followed by slow furnace-cooling at about 600°C and air-cooling. Sub-critical annealing is carried out at 735 to 785°C, followed by slow furnace-cooling.

Hardening

Grade 440 stainless steels are heated at 1010 to 1065°C, then quenched in air or oil. Oil quenching is usually carried out for heavy sections. Following this process, these steels are tempered at temperatures from 150 to 370°C, to achieve a high hardness and improved mechanical properties.

Tempering at temperatures between 425 and 565°C should be avoided as the corrosion and impact resistance properties of grade 440 tend to reduce in this range. Also, tempering of these grades at 590

 to 675°C will result in high impact resistance and loss of hardness.

Tempering :

 For maximum properties, soak at 300 F (148 C). Maximum obtainable hardness is RC 60.

Welding

During welding, grade 440 stainless steels are pre-heated at 250°C, followed by full annealing. Grade 420 filler rods can be used to achieve a high hardness weld. Grade 309 or 310 filler rods will, however, provide soft welds, having high ductility.

Machining

Grade 440 stainless steels can be easily machined in their annealed state. Machining these grades after hardening is extremely difficult, or impossible in some cases.

Best machined in the annealed condition. Tough, stringy chips can be best handled by the use of chip breakers. Carbide or ceramic tooling is recommended.

Applications

Grade 440 stainless steels find applications in the following:

• Chisels
• Surgical equipment
• High quality knife blades
• Valve seats
• Rolling element bearings
• Ball beatings
• Gage blocks
• Moulds
• Dies
• Cutlery
• Valve components
• Measuring instruments

KNIFE STEEL 440C

Collectors, investors, professional knife users, active duty military, and others will continue to specifically order fine custom handmade knives made of 440C (and other steels) based on their needs, desires, and intelligent research. 440C is a fine steel for hand knives, it’s bright, beautiful, durable, and proven, when properly processed and finished. It excels in corrosion resistance, and is many times stronger than carbon steels, while being reasonably priced. It has high wear resistance, high toughness, and extremely high finish value, which is seldom mentioned, yet extremely important. It won’t soon be replaced in the industrial and military complex as a fine, high grade martensitic stainless steel, and the machine tool industry will continue to rely upon it. There is no current replacement for 440C that has the same characteristics as this steel; if there were, and it was less expensive, it would disappear. This is how all products and their production works.

>In knives, there are only a couple steels that can compare with its beauty while exhibiting the toughness and wear resistance that 440C has. A polished 440C blade will hold its finish for many decades with a minimum of care. It’s reasonable to sharpen, it’s affordably priced and machined. This is why it simply is a popular knife steel and will remain so.