Argentum Solutions, Inc.
Sterling guidance on corrosion and materials degradation
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David C. Silverman |
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The generic term stainless steel covers a multitude of standard compositions and variations of these compositions often bearing company trade names. Compositions vary from fairly simple alloys containing iron with about 11 wt% chromium and little else beyond impurities to complex alloys containing 30 wt% chromium, substantial quantities of nickel, and a number of other elements including molybdenum, niobium (columbium), and nitrogen specifically added to improve properties. All possess two characteristics, the iron content is at least 50 wt% of the alloy and the chromium content tends to be greater than 11 wt%. If the iron content falls below 50 wt%, the alloy is often no longer considered stainless steel but becomes classified as an alloy of the major component (e.g. nickel-based alloy). All of the stainless steels can be classified into six major categories. These are
The most comprehensive numbering system to use is the UNS (Unified Numbering System). In this system, each stainless steels composition is uniquely described by one letter and 5 numbers, for example SXXXXX. The relationship between the UNS and the more common AISI system is that the first three digits of the UNS number are the three digits of the AISI number. The second two digits usually designate variations in composition. For example, 316 stainless steel (AISI) is S31600 (UNS). The low carbon grade of 316 stainless steel, 316L (AISI) is S31603 (UNS). In this case the "L" in the AISI designation stands for "low carbon", not lead. Low carbon grades of 304, 316, 317, etc. are specified as 304L, 316L, and 317L, etc. in the AISI classification system. Society of Automotive Engineers (SAE), military, and federal classifications also exist that tend to standardize on mechanical properties in addition to composition. All stainless steels have relatively high ultimate tensile strength, high work-hardening rate (especially for the austenitic stainless alloys), and high ductility. These factors explain the tendency for stainless steels to be prone to form a built-up-edge on the tool. The chips exert a high pressure on the nose of the tool at the cutting edge which when combined with the higher temperatures at the chip-tool interface can cause pressure welding of the chip to the tool. The low thermal conductivity contributes to heat build-up. Cutting fluids are essential for maintaining high production. Cutting speeds lower than for steel may be required for non-free machining stainless steel. Their general corrosion resistance tends to be far better than carbon and low alloy steels. Austentic Stainless Steels (3XX or S3XXXX) Some General Machining CharacteristicsThe austenitic stainless steels cannot be hardened to form martensite by quenching. Instead, cold working of the alloy can make them significantly stronger and harder. All of the austenitic grades can be the most difficult to machine relative to other stainless steels. They tend to have a higher work hardening rate than do other stainless steel alloys. This attribute can make machining more difficult by creating a hard surface region through which the cutting edge must penetrate especially as the cutting process proceeds. Some of the problems that can occur are (1) tools will often run hotter creating a tendency to form a built-up edge, (2) chips can be stringier and tangle, (3) chatter will be more likely if the tool rigidity is inadequate, and (4) cut surfaces will become work hardened and more difficult to machine if cutting is interrupted or if the feed rate is too low. Classification The more common, non-specialty grades of austenitic stainless steels have a first numeral of "3", whether AISI (XXX) or UNS number SXXXXX. The parent of the austenitic class is AISI 302. Since these alloys can be strengthened by work hardening, they tend to be used in structural applications where some corrosion resistance is required. The following table shows alloy numbers (AISI designations and UNS numbers), compositions, and the characteristics which tend to define them in classes. Only the more common members are included.
Manganese Substituted Stainless Steels (2XX or S2XXXX) The AISI 200 series of stainless steels (S2XXXX) was developed during the 1930’s. These alloys were used during the Korean War to preserve nickel. About 4 wt% of the nickel is replaced by manganese with small (<0.25 wt%) nitrogen for strength. The alloys are austenitic. AISI 201 (16-18 wt% chromium, 3.5-5.5 wt% nickel, 5.5-7.5 wt% manganese) and AISI 202 (17-19 wt% chromium, 4-6 wt% nickel, and 7.5-10 wt% manganese) are common grades. These grades may be a bit easier to machine than their 3XX counterparts because of the addition of manganese. There are several additional alloys that carry UNS numbers such as UNS20300 but are not listed with an AISI designation.A number of stainless steel alloys in this classification have the trade name "Nitronic". These alloys tend to have higher strength and good wear resistance. Some examples are Nitronic 50 (21.2 wt% chromium, 12.5 wt% nickel, 2.5 wt% molybdenum, and 5 wt% manganese) with corrosion resistance better than AISI 316 and Nitronic 60 (17.0 wt% chromium, 8.5 wt% nickel, 4 wt% silicon, and 8 wt% chromium) with excellent wear resistance. Ferritic Stainless Steels (4XX or S4XXXX) As a whole, ferritic alloys tend to be the easiest of the stainless steel alloys to machine. They will not harden when heated to elevated temperatures and quenched because ferrite and not martensite form when cooled. Carbon contents tend to be low, usually below 0.15 wt% except for AISI 442 and AISI 446 which have nominal contents of 0.20 wt%. The lower chromium ferritic alloys such as AISI 405 and AISI 430 are generally easier to machine than other non free machining alloys. A number of the alloys have free machining versions, for example AISI 430F (additional sulfur) and AISI 430FSe (additional sulfur and selenium) are free machining versions of AISI 430. Increasing the hardness level of any of these alloys tends to decrease their relative machinability but not to the point that they cannot be machined relatively easily for stainless steel.Classification The Ferritic stainless steels all have the number "4" as the first numeral whether AISI (XXX) or UNS number UNS LXXXXX (L = letter, X = digit). All of these alloys contain little to no nickel meaning that the austenite phase is not stabilized at lower temperatures. Not all AISI 4XX stainless steels are ferritic as martensitic stainless steels share the "4". Those alloys that are ferritic cannot, as a rule, be strengthened by heat treatment. The three standard grades are classified by chromium content as shown in the following table. In addition, a number of specialized grades exist, a couple of the more common of which are included in the table. When in doubt, the UNS number (letter followed by five digits) will uniquely identify the alloy.
Martensitic Stainless Steels (4XX or S4XXXX ) Martensitic stainless steels have high hardenability because of their alloy content. That is, a greater thickness will be hardened relative to plain carbon steels under given quenching conditions. When they are heated to high temperatures and quenched they will form a martensite phase which is extremely hard. The carbon content varies from about 0.15 wt% to about 1.2 wt% depending on alloy. Annealed hardness increases with increasing carbon level. Machinability decreases as the carbon content increases. The annealed low carbon, straight chromium grades (for example AISI 403 and AISI 410) are generally easier to machine than other non-free machining alloys. The alloys are often machined in the annealed condition. Relative to other stainless steels, the machinability of martensitic alloys is generally between ferritic (easier) and austenitic (more difficult). Within the class, machinability difficulty increases in the order AISI 410, AISI 420, AISI 440 in which the nominal carbon level increases from 0.15 wt% in AISI 410 to about 0.20 wt% in AISI 420 to a range of 0.95 wt% to 1.2 wt% in AISI 440C. Differences in machinability between free machining and non free machining versions become less as the carbon level increases. The reason is that abrasive chromium carbide tends to form at the higher carbon levels. If the alloy contains nickel, it tends to be harder to machine than does its non-nickel counterpart.Classification Martensitic stainless steels can be placed into two groups which are associated with the mechanical properties, low carbon alloys with maximum hardness of about 45 on the Rockwell C scale and higher carbon alloys which can be hardened to 60 on the Rockwell C scale. For simplicity, the alloys can be classified as falling on one side or the other of an 0.15 wt% carbon level. One other grade classification has been established, one that contains the nickel-bearing martensitic alloys. All of the alloys carry the AISI 4XX designation (UNS S4XXXX) just like the ferritic alloys. The "XX" portion of the numbers is different. The following table lists some of the most common grades.
Duplex Stainless Steels Duplex stainless steels contain a mixture of ferrite and austenite crystallites. They have good ductility and toughness along with usually good corrosion resistance. Strength can be increased by cold working. Hardness is often comparable to austenitic alloys containing nitrogen. Machinability is somewhat limited by their high annealed strength level. In terms of tool life, these alloys tend to be somewhat more machinable than stainless steel alloys hardened by the addition of nitrogen but somewhat less machinable than austenitic alloys of normal composition. No enhanced machining version of duplex stainless steel alloys exists.Classification This class of stainless steels has begun to find more widespread use as a material of construction in various process industries because of resistance to chloride stress corrosion cracking. Since these alloys contain a mixture of ferrite and austenite, they tend to have features that mimic a combination of both types of alloys. Their resistance to stress corrosion cracking is far better than that of austenitic stainless steels but not quite as good as that of ferritic stainless steels. Their toughness is superior to ferritic stainless steels but inferior to austenitic stainless steels. As mentioned above, their strength is better than austenitic stainless steels. The chromium content of these steels tends to be at three nominal levels, 18 wt%, 22 wt%, and 25 wt%. The nickel content tends to lie between 3 wt% and 6 wt% functioning mainly to control the microstructure. All of the alloys contain molybdenum for resistance to localized corrosion. Some of the alloys contain nitrogen. There is no AISI numbering system for these alloys other than AISI 329. The alloys tend to be named by their manufacturer. Some members of this class are specified in terms of an ASTM or ASME specification. The UNS number, if available, uniquely designate alloys in this class and enable easier identification. The following table shows some of these alloys and the standard to which their characteristics must conform. More exist and still more are continually being developed. The table is provided to show examples.
Precipitation Hardening Stainless Steels (6XX) Precipitation hardening stainless steels achieve high strength through a relatively low temperature treatment that can be applied after fabrication. This treatment causes precipitation of a secondary phase in the alloy that increases hardness. Some alloys require one treatment, others two. Machining and other fabrication is often done between the quenching step on the unfinished part and the aging step on the final part. These alloys do not rely on cold working or conventional heat treatments to achieve high strength. There are three groups (1) martensitic, (2) semi-austenitic, and (3) austenitic. The machinability depends on the type of alloy and its hardness. Many of the martensitic precipitation hardenable stainless steels machine in a manner comparable to AISI 304. One member of this class alloy 17-4 PH is available in a free machining version. These alloys can also be machined in an aged condition but such machining can be more difficult requiring lower cutting speeds. Machinability of the semi-austenitic alloys depends on the state of the alloy. Machinability in the annealed, austenitic state can be somewhat worse than AISI 302 which work hardens. As with the martensitic precipitation hardenable alloys, machining difficulty increases with aged hardness level. The austenitic precipitation hardenable alloys machine poorly and can require still slower cutting speeds. After aging, machining ability can degrade further.Classification Three classes of precipitation hardening stainless steels exist (1) martensitic, (2) semi-austenitic, and (3) austenitic. Most of the alloys are incorporated in the AISI high-temperature, high strength classification designated as 6XX though they are usually better known by the trade names given by their manufacturers. When a designation appears as XX-X PH, the first "XX" is the nominal chromium concentration in weight percent. The second "X" is the nominal nickel content, both in weight percent. Sometimes the PH ("precipitation hardening") appears in front of the designation. If an element appears after the name, that element is present in the alloy at a more significant (often greater than about 2 wt%) level. Just because an element is not named, does not mean that it is not present. The following table shows the designation and composition of several of the more common alloys. The table is not all-inclusive but contains some of the more commonly encountered precipitation hardening alloys.
Some additional characteristics of the three classes are as follows:
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David C. Silverman, Ph.D. - Primary Consultant |