4340 is an excellent steel for applications where strength is the most important factor, and sharpness is not as important as throwing knives and axes or blunt tools such as hammers. Knife steels typically have a carbon range between 0.7% and 1.2%, while 4340 maxes out at 0.43% carbon.
The high hardenability and resistance to fatigue make this alloy ideal for applications such as forged hydraulics and machine tools. It is often used in the automotive and aircraft industries, and in forged steel crankshafts, landing gear, axles, oil and gas extraction, gears, sprockets, and spindles.
What Is 4340 Steel?
4340 is nickel-chromium-molybdenum alloy steel with a medium carbon content of 0.38-0.43 percent.
Nickel aids in corrosion resistance and makes the steel easier to form. Nickel-containing steels remain ductile at lower temperatures, but can still be used for high-temperature applications.
Chromium greatly increases the hardenability of steel alloys and is key in improving corrosion resistance.
Molybdenum is a strong carbide former. It increases hardenability and elevated temperature strength.
Manganese, also an important ingredient in this alloy, decreases the brittleness of steel and adds strength by increasing hardenability. It assists in deoxidation and prevents the formation of iron sulfide and inclusions.
What is 4340 Steel Composition?
The composition of this alloy is: carbon 0.38-0.43%, manganese 0.6-0.8% max, phosphorus 0.035% max, sulfur 0.040% max, silicon 0.15-0.3, nickel 1.65-2%, chromium 0.7-0.9%, molybdenum 0.2-0.3%
What Are 4340 Steel Properties?
4340 has a tensile strength of 108,000 psi and a yield strength of 68,200 psi. It is very tough and attains high strengths in the heat-treated condition. It maintains good toughness, wear resistance, and fatigue strength.
To heat-treat 4340: heat it to 1550° F (843°C), then quench in warm oil.
After being heat-treated, or normalized and heat-treated, 4340 alloy can be tempered. It should be tempered twice, in two hour-long cycles, and let cool to room temperature between cycles.
The temperature at which tempering is done depends on the strength level desired. For strength levels in the 260-280 ksi range tempering should be at 450°F, for 125-200 ksi range temper at 950°F.
Do not temper 4340 steel if the desired strength is in the 220-260 ksi range. At this level tempering results in loss of impact resistance. Additionally, tempering between 480°F to 842°F should be avoided due to temper brittleness.
After forging, 4340 should be slow cooled in ashes or sand.
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This alloy has a Rockwell C hardness of 17.
Will 4340 Steel Rust?
This steel has good corrosion resistance and is often used in outdoor applications.
Acidic degradation is warded off by the addition of chromium to the alloy. The chromium creates a passive oxide layer which renders the surface electrochemically passive in the face of corrosive elements. As the chromium content of an alloy rises so does the stability of this passive layer.
Other factors in corrosion resistance are heat-treatment, as described above, and surface conditioning.
How Strong IS 4340 Steel?
4340 is high tensile steel with a tensile strength of 108,000 psi. It has a yield strength of 68,200 psi.
Tensile strength refers to the overall strength of the metal. It is measured by the pounds per square inch of force required to break a metal bar. Yield strength is the measure of how much pressure can be applied before the object is permanently deformed. The elasticity, or ability to return to its original shape after being stretched, is what is being measured with a yield strength test.
The strength of steel is measured relative to other steels. 4340 is considered one of the toughest steels for its relative strength.
4340 alloy is typically used to make heavy-duty axles, shafts, heavy-duty gears, spindles, pins, studs, collets, bolts, couplings, sprockets, pinions, torsion bars, connecting rods, crowbars, conveyor parts, etc.
Is It Easy To Sharpen 4340 Steel Blade?
Stainless steel blades are going to be a bit tougher to sharpen than a higher carbon blade. While 4340 alloy doesn’t have quite a high enough chromium content to be classified as stainless, its lower carbon content can make sharpening more difficult.
If you are already familiar with using whetstones to sharpen your blades a good option is a diamond grit whetstone. Use a light pressure so that you don’t strip away the diamond layer. It will take about 5 to 10 minutes to sharpen a stainless steel blade with this method.
Another option is to use a manual system with preset angles. Manual systems range from relatively inexpensive to premium features (and price). You can put a nice edge on a blade, even with no previous experience with this tool, although it does take extra time, up to 20 minutes or so.
You can also use sandpaper to sharpen your steel blades. Be sure to use silicon carbide rather than aluminum oxide. Get a range of grits, from 60 up to 1200. Lay the sandpaper on a flat, firm surface. You can quickly form an edge with the 60 grit, then gradually work your way up in fineness to smooth out the edge.
What Are The Best 4340 Steel Knives in the Market
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This particular steel alloy is outstanding for tools that need to be tough and don’t need to hold a particularly sharp edge, such as a throwing ax. Or for making blunt tools such as hammers or a hammer eye drift.
For knife making consider tool grade steels, which can balance the benefits and drawbacks of carbon steels vs stainless steels. Typical tool steel used for knife-making includes A2, D2, and M2. A2 is quite tough but at the cost of hardness. D2 is not as tough but has better edge retention and corrosion resistance. M2 borders on brittle but maintains a perfect edge.
The 400 series of stainless steels are a safe bet for good edge retention and are usually used to make quality kitchen knives.
If you’re making a blade that needs to last and take a beating, such as a bowie knife or a machete, consider using carbon steel with a content of 0.45-0.60%, as these have adequate hardness without becoming brittle during processing.