SS Machinability: S & P Enhancements & Benefits

[RPI_Quantum]

In looking at various stainless steels, I notice that S and P are added to "improve machinability." What is the mechanism here? And what exactly makes a material "machinable"?

 

[Gokul43201]

At the basic level (that's as far as my knowledge goes), the addition of S and P in austenitic (or other) SS, leads to the formation of brittle non-metallic particles (primarily manganese sulfide/phosphide...actually, them's the only ones I remember) which help chips to break off. This makes it easier to take deeper cuts out of the metal.

This is based on a foggy recollection of something I read a long time ago...so do not treat it as authoritative.

 

[PerennialII]

Have a similar vague recollection ... would add oxides to the batch of inclusions used in improving machinability. If am not completely mistaken, some of the inclusions, in addition to improving chipping, have tool lubricating effects (as solid lubricants). The key (if memory serves) is to get inclusions of appropriate "softness", which as such help rather than deteriorate machinability (and other properties naturally, or try to limit such deterioration to minimum), and the development of such inclusions requires some specific steel making process "fine tuning".

 

[Astronuc]

Gokul and Perennial are correct. I believe MoS₂ is also one factor, particularly in 316S, where the S denotes addition of sulphide.

While adding sulfur improves the machinability of austenitic SS, it also reduces fracture toughness, raised the NDTT (Nil Ductility Transition Temperature), and can also be a site for anaerobic MIC (Microbial Influence Corrosion).

Nuclear grade 304 and 316 have extremely low levels of S and P.
Most stainless steels, particularly 300 series, usually have 0.045% P and 0.03% S.

It is better to use a good lubricant and sharp cutting tools to machine SS, rather than use a grade that is more 'machinable'. Many years ago, an SS coupling on a transfer line to a tankcar loaded with HF sprung a leak. The coupling was made of 316SS and it fractured during a cold period, when the temperature was below the nil-ductility transition temperature. The fracture occurred as a result of 'normal' operations in which the coupling was struck with a hammer or mallet in order to loosen it. This fact was never considered in the coupling design nor material selection. Engineers must expect the unexpected, and anticipate that which one would not normally anticipate.

 

[Gokul43201]

Many years ago, an SS coupling on a transfer line to a tankcar loaded with HF sprung a leak. The coupling was made of 316SS and it fractured during a cold period, when the temperature was below the nil-ductility transition temperature. The fracture occurred as a result of 'normal' operations in which the coupling was struck with a hammer or mallet in order to loosen it. This fact was never considered in the coupling design nor material selection. Engineers must expect the unexpected, and anticipate that which one would not normally anticipate.

Hmmm...didn't the same thing happen with the Titanic ?

 

[PerennialII]

It is better to use a good lubricant and sharp cutting tools to machine SS, rather than use a grade that is more 'machinable'. Many years ago, an SS coupling on a transfer line to a tankcar loaded with HF sprung a leak. The coupling was made of 316SS and it fractured during a cold period, when the temperature was below the nil-ductility transition temperature. The fracture occurred as a result of 'normal' operations in which the coupling was struck with a hammer or mallet in order to loosen it. This fact was never considered in the coupling design nor material selection. Engineers must expect the unexpected, and anticipate that which one would not normally anticipate.

Talk about a bummer, wouldn't have thought would influence the behavior of an austenitic SS with 'machinability' tweaking that much with respect to fracture toughness (or well, have always thought the resulting decreases in toughness are around 5-30% or so), or introduce NDTT resulting transition behavior in the first place.

 

[Astronuc]

Talk about a bummer, wouldn't have thought would influence the behavior of an austenitic SS with 'machinability' tweaking that much with respect to fracture toughness (or well, have always thought the resulting decreases in toughness are around 5-30% or so), or introduce NDTT resulting transition behavior in the first place.

Gokul you are correct about Titanic. While it was not stainless steel, but a marine structural steel, sulfur and phosphorus contributed to the failure, and the fact that the steel was low in Mn, which would have combined with some of the sulfur and improved the ductility.

A detailed analysis of the pieces revealed the ship's steel plating was of a variety that loses its elasticity and becomes brittle in cold or icy water, leaving it vulnerable to dent-induced ruptures. Furthermore, the rivets holding the hull together were much more fragile than once thought. It is unknown if stronger steel or rivets could have saved the ship.

The samples of steel rescued from the wrecked hull were found to have very high content of phosphorus and sulphur (four times and two times as high as common for modern steels), with a manganese-sulphur ratio of 6.8:1 (compare with over 200:1 ratio for modern steels). High content of phosphorus initiates fractures, sulphur forms grains of iron sulphide that facilitate propagation of cracks, and lack of manganese makes the steel less ductile. The recovered samples were found to be undergoing ductile-brittle transition in temperatures of 32 °C (for longitudinal samples) and 56 °C (for transversal samples—compare with transition temperature of −27 °C common for modern steels—modern steel would become as brittle between −60 and −70 °C). The anisotropy was likely caused by hot rolling influencing the orientation of the sulphide stringer inclusions. The steel was probably produced in the acid-lined, open-hearth furnaces in Glasgow, which would explain the high content of phosphorus and sulphur, even for the times.

http://en.wikipedia.org/wiki/RMS_Titanic#The_rediscovery_of_Titanic

 

[PerennialII]

... "for laughs" a few years in our lab we did a reanalysis of the Titanic ductile to brittle transition impact data, and got similar values for T27J transition temperatures. One of those times after which everyone was trying to figure out how an Earth it could float even that far in the first place, "a ship made of glass" by 'usual standards' .

 

[Astronuc]

Yeah, after learning about the quality of the steel, I wonder if they used cheap steel on purpose. Certainly back then, they did not know as much as we know now about materials.

In fact, anyone who has taken a materials course or more likely a course in fracture mechanics probably has heard about the infamous 'Liberty' cargo ships which split apart while sailing across the North Atlantic.

Probably wasn't until after WWII that materials science really took off.

 

[RPI_Quantum]

Hey everyone, thanks!

 

[PerennialII]

You're welcome RPI! Hope we don't distort the thread too bad .

Yeah, after learning about the quality of the steel, I wonder if they used cheap steel on purpose. Certainly back then, they did not know as much as we know now about materials.
In fact, anyone who has taken a materials course or more likely a course in fracture mechanics probably has heard about the infamous 'Liberty' cargo ships which split apart while sailing across the North Atlantic.
Probably wasn't until after WWII that materials science really took off.

Yeah, impurity control at that time must be off the scale and rimmed steel and all, such steels still come up occationally ... when doing failure analyses .
Actually while we're at it there was a piece in JOM a while back which is online:
http://www.tms.org/pubs/journals/JOM/9801/Felkins-9801.html

Table II. The Composition of Steels from the Titanic, a Lock Gate, and ASTM A36 Steel
C Mn P S Si Cu O N MnS: Ratio
Titanic Hull Plate 0.21 0.47 0.045 0.069 0.017 0.024 0.013 0.0035 6.8:1
Lock Gate* 0.25 0.52 0.01 0.03 0.02 — 0.018 0.0035 17.3:1
ASTM A36 0.20 0.55 0.012 0.037 0.007 0.01 0.079 0.0032 14.9:1
*Steel from a lock gate at the Chittenden ship lock between Lake Washington and Puget Sound, Seattle, Washington.

Composition
During an expedition to the wreckage in the North Atlantic on August 15, 1996, researchers brought back steel from the hull of the ship for metallurgical analysis. After the steel was received at the University of Missouri-Rolla, the first step was to determine its composition. The chemical analysis of the steel from the hull is given in Table II. The first item noted is the very low nitrogen content. This indicates that the steel was not made by the Bessemer process; such steel would have a high nitrogen content that would have made it very brittle, particularly at low temperatures. In the early 20th century, the only other method for making structural steel was the open-hearth process. The fairly high oxygen and low silicon content means that the steel has only been partially deoxidized, yielding a semikilled steel. The phosphorus content is slightly higher than normal, while the sulfur content is quite high, accompanied by a low manganese content. This yielded a Mn:S ratio of 6.8:1—a very low ratio by modern standards. The presence of relatively high amounts of phosphorous, oxygen, and sulfur has a tendency to embrittle the steel at low temperatures.
Davies7 has shown that at the time the Titanic was constructed about two-thirds of the open-hearth steel produced in the United Kingdom was done in furnaces having acid linings. There is a high probability that the steel used in the Titanic was made in an acid-lined open-hearth furnace, which accounts for the fairly high phosphorus and high sulfur content. The lining of the basic open-hearth furnace will react with phosphorus and sulfur to help remove these two impurities from the steel. It is likely that all or most of the steel came from Glasgow, Scotland.
Included in Table II are the compositions of two other steels: steel used to construct lock gates at the Chittenden Ship Lock between Lake Washington and Puget Sound at Seattle, Washington,8 and the composition of a modern steel, ASTM A36. The ship lock was built around 1912, making the steel about the same age as the steel from the Titanic.
Metallography
Standard metallographic techniques were used to prepare specimens taken from the hull plate of the Titanic for optical microscopic examination. After grinding and polishing, etching was done with 2% Nital. Because earlier work by Brigham and Lafrenière9 showed severe banding in a specimen of the steel, specimens were cut from the hull plate in both the transverse and longitudinal directions. Figure 2 shows the microstructure of the steel. In both micrographs, it is apparent that the steel is banded, although the banding is more severe in the longitudinal section. In this section, there are large masses of MnS particles elongated in the direction of the banding. The average grain diameter is 60.40 µm for the longitudinal microstructure and 41.92 µm for the microstructure in the transverse direction. In neither micrograph can the pearlite be resolved. For comparison, Figure 3 is a micrograph of ASTM A36 steel, which has a mean grain diameter of 26.173 µm.
Figure 4 is a scanning electron microscopy (SEM) micrograph of the polished and etched surface of steel from the Titanic. The pearlite can be resolved in this micrograph. The dark gray areas are ferrite. The very dark elliptically shaped structure is a particle of MnS identified by energy-dispersive x-ray analysis (EDAX). It is elongated in the direction of the banding, suggesting that banding is the result of the hot rolling of the steel. There is some evidence of small nonmetallic inclusions and some of the ferrite grain boundaries are visible.
...
Comparing the composition of the Titanic steel and ASTM A36 steel shows that the modern steel has a higher manganese content and lower sulfur content, yielding a higher Mn:S ratio that reduced the ductile-brittle transition temperature substantially. In addition, ASTM A36 steel has a substantially lower phosphorus content, which will also lower the ductile-brittle transition temperature. Jankovic8 found that the ductile-brittle transition temperature for the Chittenden lock gate steel was 33°C. The longitudinal specimens of the Titanic hull steel made in the United Kingdom and those specimens from the Chittenden lock steel made in the United States have nearly the same ductile-brittle transition temperature.

http://www.writing.eng.vt.edu/uer/bassett.html

The failure of the hull steel resulted from brittle fractures caused by the high sulphur content of the steel, the low temperature water on the night of the disaster, and the high impact loading of the collision with the iceberg.

So the state of the art of the time is somewhat lacking in nowadays terms.

... I think I'll use the opportunity to update some of my "introduction to cleavage" slides ... Titanic "sells" better than the Liberty ships .

 

[FredGarvin]

The liberty ships were still an impressive manufacturing achievement. I have to marvel at the incredibly simple fix they came up with to help the problem. I think it's one of the few cases where a band-aid actually did the trick.