How Chemistry Determines Machinability- Manganese Sulfides

March 12, 2015

Flank wear is the “normally expected” failure mode for tools to fail when machining steels.

The volume fraction of Manganese Sulfides is a determinant of the tool’s wear rate. “The wear rate of high speed steel tools decreases rapidly up to about one percent volume fraction of MnS and then levels off to a constant wear rate as the volume fraction is increased.“-Roger Joseph and V.A.Tipnis, The Influence of Non-Metallic  Inclusions on the Machinability of Free- Machining Steels.

Manganese and Sulfur have a powerful effect in reducing flank wear on HSS tools

Manganese and Sulfur have a powerful effect in reducing flank wear on HSS tools

As sulfur rises beyond 1% volume fraction, surface finish improves, chips formed are smaller with less radius of curvature, and the friction force between cutting tool and chip decreases due to lower contact area.

Manganese sulfides are a separate internal phase.

Manganese sulfides are a separate internal phase.

How does Manganese Sulfide improve the machinability?

  • The MnS inclusions act as “stress raisers” in the shear zone to initiate microcracks that subsequently lead to fracture of the chip;
  • MnS inclusions  also deposit on the  wear surfaces of the cutting tool as “Built Up Edge (BUE).”
  • BUE reduces friction between the tool and the material being machined. This contributes to lower cutting temperatures.
  • BUE mechanically separates or insulates the tool edge from contact with work material and resulting heat transfer.

This is why resulfurized steels in the 11XX and 12XX series can be cut at much higher surface footage than steels with lower Manganese and Sulfur contents.

More info about Manganese in steel HERE


Killed Steel

July 20, 2010

Some things you want to have bubbles, some you don’t.

Usually, Bubbles are good.

In beermaking, yeast consumes the sugars in the wort and convert them into CO2 gas bubbles- carbonation.

In steel making the main reaction is the combination of Carbon in the melt with Oxygen to form a gas. At the high temperatures involved, this gas is very soluble in the molten bath.

If the Oxygen that is available for this chemical reaction isn’t completely removed before the steel is cast the gases will continue to be forced out of the melt during solidification, resulting in porosity in the steel.

Bubbles and where the gas goes can be important in your steel part.

In order to control the evolution of gas, chemicals called deoxidizers are added to the steel. These chemicals, Silicon or Aluminum, Vanadium, Columbium, Niobium scavenge the available oxygen in the molten steel, react chemically to form solid oxide particles dispersed throughout the steel, rather than bubbles of Carbon Dioxide.

The amount and type of deoxidizer added determines the type of steel. If sufficent deoxidizers are added, no gas is evolved from the solidifying steel, and the steel is said to be “killed.” The ingot drawing labelled number 1 shows a fully killed (deoxidized) steel showing only a shrinkage cavity, and no bubbles or porosity. ( This shrinkage cavity would be cropped off in normal rolling practice.)

Because gas is still evolving, this beer is NOT KILLED.

Killed steel has more uniform chemical composition and properties than rimmed, semi-killed, or non-killed steels, and generally less segregation. The uniformity of killed steel and and its freedom from porosity makes these steels more suitable for critical components and for applications involving heat treatment.

Killed steels generally contain 0.15-.35 weight percent Silicon as a deoxidizer, and may contain  some of the other elements as mentioned above. These other elements may be used as deoxidizers or as grain refiners.

Steel grades with a Carbon maximum of 0.30 weight % and above, and all alloy steels are typically provided as “killed steels.”

Free machining steels such as 12L14, 1215, and some 11XX series steels are not “killed” with Silicon, Aluminum, etc., due to their deleterious effects on tool life and machinability. The high amounts of Manganese  in these steels form Manganese Sulfides to promote machinability, and also the Manganese scavenges excess Oxygen, preventing  evolution of CO2.

Killed steel is specified so your critical parts won't have bubbles in them.

Killed steel- for critical parts. Non-killed beer for critical  after work down time.

Cheers!

Beer Bubbles Photo Credit

Ingot scan from a handout in my files originally after Making Shaping and Treating of Steel.

 Beer Head Photo Credit

Bread with Holes

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The Difference Between Microalloy And Regular Alloy Steels

May 5, 2010

As machinists, we seldom encounter microalloy steels. but what do we need to know?

  1. Microalloy steel is manufactured like any other, but the chemical ingredients added at the initial  melt of the steel  to make it a microalloy include elements like Vanadium, Columbium (sorry, Niobium for us IUPAC  purists), Titanium, and higher amounts of Manganese and perhaps Molybdenum or Nickel.
  2. Vanadium, Columbium Niobium, and Titanium are also grain refiners and aggressive Oxygen scavengers, so these steels tend to also have a very fine austenitic grain size.
  3. In forgings, microalloy steels are able to develop higher mechanical properties (yield strengths greater than say 60,000 psi) and  higher toughness as forged by just cooling in air or with a  light mist water spray.
  4. Normal alloy steels  require a full austenitize, quench and temper heat treatment to develop properties greater than as rolled or cold worked.

Since microalloyed steels are able to get higher properties  using forging process heat- rather than an additional heating quenching tempering cycle- they can be less expensive to process to get improved mechanical properties.

 The developed microstructure ultimately makes the difference. The  microstructure developed in the steel depends on the grade and type.

Tempered martensite for normal alloys.

  • Normal alloy steels require a transformation to martensite  that is then tempered in order to achieve higher properties.

Bainite comparable hardness improved toughness.

  • Microalloy steel precipitates out various nitirides or carbides and may result in either a very fine ferrite- pearlite microstructure or may transform to bainite.

For machinists, if the steel is already at  its hardest condition, the microalloyed microstructure of either ferrite pearlite or bainite  is less abrasive than that of a fully quench and tempered alloy steel.

P.S. The non- martensitic structures also have higher toughness.

We don’t tend to machine prehardened steels in the precision machining industry, but if you ever are part of a team developing a process path for machining forgings, or finish cuts after induction hardening, these facts might be good to know.

Martensite.

Georges Basement Bainite 1000X

 

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5 Facts About Manganese in Steel

February 16, 2010

The role of Manganese in steel in our precision machining shops.

Manganese ore like this comes from Turkey.

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.

  1. The effectiveness of Manganese in increasing mechanical properties depends on and is proportional to the carbon content of the steel.
  2. 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.
  3. Manganese is also an active deoxidizer, and is less likely to segregate than other elements.
  4. 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.
  5. 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.

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Carbon Equivalent And Weldability

December 29, 2009

The weldability of steels is influenced primarily by the carbon content. At higher carbon levels, steels may need either pre- or post- weld heat treatment in order to prevent stress build up and weld cracking.

Generally speaking, if the Carbon Equivalent (CE) is 0.35 or below, no pre- or post- weld  thermal treatment  is needed. In our experience with maintenance welding, we have found that preheating was beneficial between 0.35 and 0.55 CE. Above 0.55 CE we usually both pre- and post- weld heated to relieve stress and prevent cracking.

So CE= .35 max.

However the other elements that are contained in the steel also have an effect on the steel’s “carbon equivalence.” These additional elements can really add up in scrap fed electric arc  furnace steels that now predominate in our market.

Electric Arc Furnaces are predominately scrap fed.

Photo credit.

Here are two formulas for calculating Carbon Equivalents.

CE=%C+(%Mn/6)+(%Cr+%Mo+%Va)/5 + (%Si+%Ni+%Cu)/15

This is the first formula I learned when I took over metallurgical support for  maintenance ‘back in the day.’

In this formula you can see that 6 points of Manganese are approximately equal to one point of Carbon.  5 points of Chrome, Moly or Vanadium are roughly equal to a point of Carbon, while it takes about 15 points of Silicon, Nickel or Copper to get about the same effect as one point of Carbon.

The GE formula for Carbon Equivalency is CE= C+(Mn/6)+(Ni/20)+(Cr/10)+(Cu/40)+(Mo/50)+(Va/10). If this is less than .35 max, you should have no need to pre or post weld thermal treat in most cases.

As long as CE is no more than .35, you probably won’t need to preheat or post weld stress relieve your welded parts. above .35 CE, you may need either or both depending on section thickness and CE.

* (I) added (extra parentheses) to keep (the terms) clear in (this post); no (scathing rebukes) from (math teachers) please!

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