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Functional tools are stiff and strong, to transfer force and feedback without excessive flexing or absorption. Durable tools are resilient and tough, to flex instead of bend, and bend instead of break. The ideal metal is stiff, strong, resilient, tough, corrosion resistant, inexpensive, and available.
The metals in common use today are 301 & 302 full hard stainless steel, 301 high yield stainless steel, 420 stainless steel, and carbon spring steel. There are many other metals that look good on paper, but are too expensive or difficult to obtain to be practical.
301 and 302 stainless steels are austenitic iron chromium nickel alloys used for springs that derive all of their strength from cold rolling. Heating anneals the metal to a yield strength of 30ksi, ruining the strength permanently. Only cold machining processes may be used. 301 full hard is soft enough to be stamped, and makes low cost tools with a yield strength of 140ksi. 302 full hard contains 1-2% more chromium and nickel than 301 full hard, for increased corrosion resistance and decreased work hardening, which may improve stamping quality. 301 high yield must be cut with a cold machining process such as PCM, water jet, or wire EDM, and makes high quality tools with a yield strength of 260ksi.
420, 420HC, 440A, and 440C stainless steels are martensitic iron chromium alloys used for cutlery that derive most of their strength from heat treatment. Carbon content, strength, brittleness, and cost increase in the order: 420 → 420HC → 440A → 440C. Peterson "Government Steel" is 420 stainless steel. Leatherman multitool steel is 420HC. No tensile strength values are available for heat treated 420HC. 420 and 420HC are soft enough to be stamped before heat treatment. 440A and 440C are too hard to be stamped easily. Thin tools may warp when heat treated, so it is safer to cold machine metal that has already been heat treated. A jig to hold a stack of tools under compression during heat treatment may prevent warping.
17-7PH stainless steel is a precipitation hardening iron chromium nickel aluminum alloy used for high performance springs that derives part of its strength from cold rolling and part from heat treatment. The alloy is similar to 301 with about 1% aluminum added. Heat treatment of cold rolled condition C 17-7PH to condition CH900 results in a yield strength of 260ksi, the same as 301 high yield, but with a lower ultimate tensile strength, and the same restriction to cold machining processes. Carpenter Custom 465 and 475 are respectively the toughest and strongest precipitation hardening stainless steels.
1060, 1075, & 1095 carbon spring steels are iron carbon alloys that derive most of their strength from heat treatment. The last two digits signify the fractional percentage of carbon, thus 1095 is 0.95% carbon. As carbon content increases, strength increases and toughness decreases. The variability of heat treatment makes it difficult to obtain yield strengths for carbon spring steels.
Carbon spring steel can rust within days if exposed to high humidity, salt, acid, oxidizers, food, or sweat, can cause stainless steel to rust on contact, and creates an unpleasant odor when handled. Rust causes messy stains, handle delamination, and metal disintegration. Although the price/performance ratio is tempting, carbon spring steel is not recommended.
Titanium alloys are low density, nonferrous, nonmagnetic, corrosion resistant metals, less likely to trigger metal detectors, making them desirable for covert operations. Titanium Grade 5 / Ti6Al4V / Ti-6-4 is the most common alloy. Titanium R56620 / Ti-6Al-6V-2Sn / Ti-6-6-2 is about 15% stronger than Ti-6-4. Condition STA (Solution Treated and Aged) Ti-6-4 is about 15% stronger than Ti-6-4. Condition STA Ti-6-6-2 is about 35% stronger than Ti-6-4.
Young's modulus is 16Mpsi for titanium alloys and 30Mpsi for steel, so titanium alloys are half as stiff as steel. Compared by yield strength, titanium alloys range from half to two thirds as strong as the best steel. Titanium tools are made thicker to compensate for lower stiffness and strength, and are thus unsuitable for narrow keyways typical of high security locks.
MP35N and Elgiloy / Phynox are cobalt nickel chromium molybdenum superalloys used for Swiss watch springs and high performance parts in corrosive environments. When properly strain hardened and heat treated, they can reach a yield strength of 290ksi. Young's modulus for MP35N is 34Mpsi, 13% stiffer than the 30Mpsi of steel and Elgiloy / Phynox, which contains some iron. Superalloys are expensive and difficult to obtain.
This data was culled from numerous sources and is highly variable depending on the metal supplier, so consider it a rough estimate and defer to supplier specifications. Some metals like carbon spring steel have a large variation in yield strength due to heat treatment and are not well represented.
|1ksi = 1kpsi = 1000 psi = 1000 lb/in²||1MPa = 1N/mm²|
|Elgiloy / Phynox H560 extra hard after heat treatment||297ksi||2050MPa||319ksi||2200MPa|
|MP35N 53% cold reduction, aged 1000°F 4 hours||290ksi||2000MPa||300ksi||2070MPa|
|Carpenter Custom 475 5% cold work, condition H975||284ksi||1958MPa||294ksi||2027MPa|
|440C stainless steel tempered 400°F||276ksi||1900MPa||294ksi||2030MPa|
|Carpenter Custom 475 condition H975||265ksi||1827MPa||287ksi||1979MPa|
|301 high yield stainless steel cold rolled||260ksi||1790MPa||280ksi||1930MPa|
|17-7PH stainless steel condition CH900||260ksi||1793MPa||265ksi||1827MPa|
|Carpenter Custom 465 condition H900||245ksi||1689MPa||260ksi||1793MPa|
|Elgiloy / Phynox H560 extra hard before heat treatment||239ksi||1650MPa||261ksi||1800MPa|
|440A stainless steel tempered 400°F||229ksi||1579MPa||293ksi||2022MPa|
|420 stainless steel tempered 400°F||221ksi||1524MPa||252ksi||1737MPa|
|17-7PH stainless steel condition RH950||220ksi||1517MPa||235ksi||1620MPa|
|420 stainless steel tempered 1100°F||170ksi||1172MPa||200ksi||1379MPa|
|Titanium R56620 / Ti-6Al-6V-2Sn / Ti-6-6-2 condition STA||170ksi||1172MPa||180ksi||1241MPa|
|1095 carbon steel tempered 400°F||152ksi||1048MPa||216ksi||1489MPa|
|Titanium Grade 5 / Ti-6Al-4V / Ti-6-4 condition STA||145ksi||1000MPa||160ksi||1103MPa|
|Titanium R56620 / Ti-6Al-6V-2Sn / Ti-6-6-2 annealed||145ksi||1000MPa||155ksi||1069MPa|
|301 and 302 full hard stainless steel cold rolled||140ksi||965MPa||185ksi||1275MPa|
|Titanium Grade 5 / Ti-6Al-4V / Ti-6-4 annealed||126ksi||869MPa||134ksi||924MPa|
|301 and 302 annealed stainless steel||30ksi||205MPa||75ksi||515MPa|
Several references assert that rigidity is absolute inflexibility, and stiffness is relative inflexibility. There are no references advising the use of rigid over stiff when comparing materials. There are no English language equivalents for stiffer and stiffest - rigider and rigidest are not words. The phrases higher elastic modulus and more rigid are poor substitutes. Given the lack of reasonable options, stiffness shall be used for objects and materials.
Most companies specify ultimate tensile strength instead of yield strength when referring to their tools, because it is a bigger number and looks more impressive. This is misleading, because yield strength determines how much force can be applied without deforming the metal. Unfortunately, not all metal is rated by yield strength. Testing tensile strength is difficult on thin samples.
The Rockwell C hardness (HRC) scale is commonly used to specify steel hardness, but the test does not work on thin samples. The Vickers hardness (HV) test measures steel hardness of thin samples, and is then converted to HRC. The Crucible Steel Hardness Conversion Table correlates several hardness scales including Vickers and Rockwell to approximate ultimate tensile strength in ksi. It is theoretically possible to correlate yield strength to ultimate tensile strength, but this would require extensive testing to determine the exact ratio for the chosen metals and range of hardness.
Toughness is the area under the entire stress-strain curve. Resilience is the area under the linear region of the stress-strain curve. Toughness is a combination of strength and ductility. Strength and ductility are inversely proportional. Brittleness is a lack of ductility and malleability. Strong brittle metal fractures easily. Weak ductile metal bends easily. Resilient metal flexes elastically. Tough metal flexes elastically, then bends plastically, rather than fracturing. Toughness and resilience improve durability.
The Charpy notch test is used to measure toughness, but the test does not work on thin samples. Hardness and strength are inversely proportional to toughness. 41HRC is soft enough to stamp, but makes weak tools. 59HRC is very hard, but makes brittle tools. Hardness in the range of 48-52HRC is more likely to provide a reasonable balance of toughness and strength, but experimentation is required to determine the optimum hardness.
Strain hardening and quenching increases strength and decreases ductility. Tempering decreases strength and increases ductility. Heat treatment can improve alloy performance significantly, but is complex and expensive to implement. Some alloys require heat treatment after machining, some alloys do not need heat treatment, and some alloys must not be heated at all, including during machining. Cycles of cryogenic hardening and tempering are the most advanced and expensive of the heat treatment processes for increasing the strength and toughness of some alloys. Use pretreated alloys and cold machining to simplify production.
Tempering creates a thin film of iron oxide on the surface of steel that reflects a color corresponding to the tempering temperature. The tempering color can be used to verify the tempering temperature.
Samples of Southern Specialties 301 full hard stainless steel, SouthOrd MAX 301 high yield stainless steel, and Peterson "Government Steel" were sent to a laboratory for metallurgical analysis. The chemical analysis confirms that Peterson "Government Steel" is 420 stainless steel, the most common cutlery alloy, or Carpenter TrimRite S42010, a modified 420 with 1% molybdenum for increased corrosion resistance and ductility.
The tensile tests for yield strength and ultimate tensile strength were inconclusive because the SouthOrd and Peterson samples broke at the tab where the sample is held. Given the unreliability of tensile testing on samples this thin, it is more practical to make identical picks of 420 and 301 high yield and see which take more abuse. 420 is more expensive than 301 high yield, and is rarely used for springs due to its lower expected yield strength, but it has high toughness and hardness that deserves diligent evaluation.
301 full hard
301 high yield
|Converted Rockwell C Hardness||41||51||52|
|UNS Alloy||S30100||S30100||S42000, S42010|
Swart smeked smithes, smatered with smoke, Drive me to deeth with din of here dintes.
Swich noise on nightes ne herd men never, What knavene cry and clatering of knockes.
The cammede kongons cryen after "Cole, cole!" And blowen here bellewes that al here brain brestes.
"Huf, puf," saith that oon, "Haf, paf," that other. They spitten and sprawlen and spellen many spelles,
They gnawen and gnasshen, they grones togidere, And holden hem hote with here hard hamers.
Of a bole-hide been here barm-felles, Here shankes been shakeled for the fire-flunderes.
Hevy hamres they han that hard been handled, Stark strokes they striken on a steeled stokke.
"Lus, bus, las, das," routen by rowe, Swich doleful a dreem the devil it to drive.
The maister longeth a litel and lassheth a lesse, Twineth hem twain and toucheth a treble.
"Tik, tak, hik, hak, tiket, taket, tik, tak, Lus, bus, las, das!" Swich lif they leden,
Alle clothe-meres, Crist hem give sorwe! May no man for bren-wateres on night han his rest.
-15th century anonymous, The Blacksmiths, sung by The Mediæval Bæbes on Illumination (2009)