By retarding transformation rates, moly improves the hardenability of its alloy steelgrades.
Molybdenum is an essential micronutrient, but large doses can be highly toxic. Fortunately, we don’t eat our alloy steels.
Molybdenum (“moly”) is added to constructional steels to
Improve hardenability by slowing the transformation (moving the nose of the curve to the right);
Reduce embrittlement during tempering
Enhance the creep strength of low alloy steel grades at higher temperature,
Add resistance to corrosion.
Moly does this in very low quantities, and so it is truly a “synergistic” alloying element. Typical moly additions in constructional steels are around 0.10-0.60% by weight. Moly analysis typically runs 0.20-0.30 in the low hardening 40XX grades; 0.15-0.25 in the 41XX series of alloy steels; and 0.20-0.30 in the deeper hardening 43XX and 48XX steels.
Moly has been reported in Japanese swords as far back as the 14th Century, but its first major military use was for tank armor in World War I. The French firm Schneider & Company made moly armor plate which at 25 mm was able to stop a direct hit from a shell. The prior manganese armor plate at 75mm thick was not so impervious and the reduction of steel mass by about 2/3 made the tanks with moly armor much more mobile (speed and manuverable) in combat. Today moly is an indispensable part of many aerospace and high temperature applications including rocket nozzles.
While Moly can be the only alloying element added (40XX steels) it is also used in combination with Chrome (41XX) Nickel (46XX and 48Xx, or in a triple alloy combination with Chrome and Nickel (43XX or 86XX) as well as other grades (87XX, 88XX, and grade 9310 come to mind). But where we see moly in our shops is in our M- series tool steels. That M prefix stands for Molybdenum, which gives these tools steels their characteristic high hot hardness. Moly content in M series tool steels ranges from 4.50% up to 9.50% by weight. It is the ability of these steels to resist softening at high temperatures that makes them so useful in our shops at production speeds and feeds.
The moly tool steels also have a tendency to decarburize so careful grinding and attention to details in heat treatment is critical in toolmaking and sharpening.
For more info on Molybdenum, Click on the Mindmap for Molybdenum. Photo of moly metal.
Trivia: the first commercial heatof Moly High Speed Steel was by Universal Cyclops in 1931. Grade AISI M1. They called it Motung for, you guessed it, MOly TUNGsten.
Improve abrasion resistance in higher carbon grades.
Chromium forms complex chromium-iron carbides. These carbides go into solution into austenite very slowly, so assuring a long enough heating (soak) time before quenching is very important.
In stainless steels, ~18 % chromium is typical, (303, 304 austenitics), while analyses as low as ~12 % (403, 420), and as high as 26-28% grades are available.
In non-stainless steels, chromium is essentially a hardening element. It is often used in combination with nickel (a toughening element) to produce improved mechanical properties. In combination with molybdenum, chromium contributes to higher strength at elevated temperatures.
Chromium’s principal use is in stainless steels, where its resistance to oxidation provides the protection from oxidation and corrosion.
Chromium’s decorative properties made it a favorite among automotive and motorcycle enthusiasts. Its resistance to oxidation and staining and ability to take a high polish make it an easy choice for decorative yet functional parts. Chromium’s hardness and chemical resistance makes it ideal for protecting our tools.
Chromium has several oxidation states, Hexavalent chromium (CRVI) is of concern as an industrial environmental issue. Metallic chromium is not hexavalent, but flame cutting or welding of chromium materials may release haxavalent chromium. Chromic acid used for some chrome plating applications is hexavalent. Newer environmentally acceptable chromium finishes are trivalent. (CRIII) Link.
Chromium is named for the Greek word chroma, meaning color, as its salts are brightly colored. Chromium is a constituent of rubies, and is why ruby lasers give off their characteristic red light.
Final chromium fact: your body requires chromium. Chromium in your body ranges from 6-100 ppb in blood, up to 800 ppb in various tissues. Depending on your mass, you might contain as much as 12 milligrams of chromium in your body. Reference.
The role of Manganese in steel in our precision machining shops.
Carbon is a chemical element that is the primary hardening constituent in steel. Manganese is a chemical element that is present in all commercial steels, and contributes substantially to a steel’s strength and hardness, but to a lesser extent than does carbon.
The effectiveness ofManganese in increasing mechanical properties depends on and is proportional to the carbon content of the steel.
Manganese also plays an important role in decreasing the critical cooling rate during hardening. This means that manganese helps to increase the steel’s hardenability. It’s effect on hardenability is greater than that of any of the other commonly used alloying elements.
Manganese is also an active deoxidizer, and is less likely to segregate than other elements.
Manganese improves machinability, by combining with sulfur to form an soft inclusion in the steel that promotes a steady built up edge and a place for the chip to break.
Manganese improves yield at the steel mill by combining with the sulfur in the steel, minimizing the formation of iron pyrite (iron sulfide) which can cause the steel to crack and tear during high temperature rolling.
Manganese is an important constituent of today’s steels.
Now you know a few reasons why Mn (the abbreviation for Manganese) is the second element shown on the chemical analysis report (right after carbon). It’s That Important! Mn Ore Photocredit.
While Austenitic Grain Size is a result of chemistry (composition), the changes that it evokes in our process are a result of material structure and properties, not just the chemical ‘ingredients.’
Steel that is fully deoxidized and grain refined is more sound, less susceptible to cracking and distorting, and more easily controlled in heat treat. Well worth it in final performance compared to the machinist’s increased tooling costs. Here are 5 Ways Austenitic Fine Grained steels can affect your shop:
Poorer Machinability than Coarse Grained Steels. (The hard oxides and nitrides resulting from deoxidation and grain refinement abrade the edge of tools and coatings- this is one reason that you go through more tooling on Fine Grained Steels.)
Poorer Plastic Forming than Coarse Grained Steels.
Less Distortion in Heat Treating than Coarse Grained Steels
Higher Ductility at the same hardness than Coarse Grained Steels
Shallower Hardenability than Coarse Grained Steels.
Fine Austenitic Grain Size is a result of DELIBERATELY ADDDING grain refining elements to a heat of steel. Because these grain refining elements have been added, the steel has a “Fine Austenitic Grain Size.”
In order to make steels with this Austenitic Fine Grained Structure, the steel is first deoxidized , (usually with Silicon) and then Aluminum, or Vanadium or Niobium are added. Aluminum, Vanadium, and Niobium are called grain refiners.
After the Silicon has scavenged most of the Oxygen out of the molten steel, the grain refiner is added. (In this post I’ll stick with Aluminum as the example.) The added Aluminum reacts with Nitrogen in the molten steel to form Aluminum Nitride particles. These tiny particles precipitate along the boundaries of the Austenite as well as with in the Austenite grains. This restricts the growth of the grains.
Because the deoxidation and grain refinement create hard abrasive oxide and nitride particles, they machine and process differently than coarse grained steels.
Fine Austenitic Grain Size appears on the material test report as an ASTM value of 5 or greater. Values of 5, 6, 7, 8, or “5 and finer” indicate that the material is Austenitic Fine Grained. Typically 7 or 8 was reported for the Aluminum Fine Grain steels that I certified.
The methods for determining Austenitic Grain Size are detailed in ASTM Standard E112, Standard Test Methods for determining Average Grain Size.
To get the Coarse Austenitic Grain Size Story, see our post here.
Don’t confuse hardness and hardenability. Hardness is a material property. Hardenability is a way to indicate a material’s potential to be hardened by thermal treatment.
Hardness is resistance to penetration. Hardenability describes how deep the steel may be hardened upon quenching from high temperature. The depth of hardening is an important factor in a steel part’s toughness.
The brinell test uses a 10mm hardened steel (sometimes carbide) ball and various levels of force applied over a specified time.
The width of the impressions is measured optically and averaged. (Wider impressions mean the ball penetrated deeper, thus, the material is less hard.) The Brinell hardness number is calculated by dividing the load applied by the surface area of the indentation. Prior to today’s direct reading instruments, the measured indentation diameters could be looked up on a reference chart and the corresponding Brinell hardness number given.
The Rockwell test is similar, but uses different forces and either a smaller ball indenter (Rockwell B scale ) or a diamond indenter (Rockwell C scale).
Hardenability- Jominy Test
In the Jominy test, a standard specimen is heated then water quenched from the end, and a series of rockwell hardness tests are taken in 1/16th inch increments along the length of the specimen.
It is the influence of the steel’s chemical makeup (Carbon and Alloying elements) that determine how a deeply a grade of steel will transform to martensite for a particular quenching treatment. This means that for each grade being heat treated, mechanical properties are a result of cooling rate (quench). An excellent web page on this can be found here.
So what of the difference between hardness and hardenability? Hardness is resistance to penetration under specified conditions of load and indenter. Hardenability is the ability of a steel to acheive a certain hardness at a given depth, upon suitable heat treatment and quench. Hardness can be measured in steels in any condition. Hardenability presumes that the steels will be heat treated to acheive a targeted hardness at a given depth.
One is an actual property, one is a measure of potential.
And now you know.
Web resources: Gordon England Thermal Spray Coatings Farmingdale State College School of Engineering Technologies.