Argentum Solutions, Inc.
Sterling guidance on corrosion and materials degradation
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David C. Silverman |
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Wrought steels can be classified on the basis of composition, finishing methods (e.g. hot rolled or cold rolled), or product form (bar, plate, etc.). The most widely used system designed for designating carbon and alloy steels is that of the American Iron and Steel Institute (AISI) and the Society of Automotive Engineers (SAE). They identify the various materials by composition. The two systems, while being separate, are nearly identical and rarely is a distinction made between the two. The differences have to do with eligibility in terms of tonnage and usage. Cross referencing to other classification systems can be difficult and the reader is referred to "Properties and Selection: Iron, Steels, and High Performace Alloys", ASM Handbook, Volume 1. Many of the compositions can be made into different forms (bar, sheet, rod, etc.). The specification of these forms is usually governed by other standards such as those developed by ASTM (American Society for Testing and Materials) or by the military (military specification identified as MIL-XXXX). Thus, an ASTM specification for mechanical tubing, ASTM A519 for example, can encompass a number of compositions (SAE or AISI grades). The compositions or tolerances (ranges) may be more restricted than those implied by the SAE/AISI numbers for the alloys included in those standards. The SAE/AISI designation system specifies each steel by four digits (1XXX to 9XXX). Each of these numbers may have letters associated with it. The numbers specify the constituent ranges in weight percents. Letters preceding the four digits show the steel-making method if A, B, C, D, or E. If a letter is omitted, the production method is the open hearth method. If the letter "M" precedes the number, the alloy is "merchant" quality. This quality is the least restrictive descriptor and steel barstock of this quality is usually used in the production of non-critical parts of bridges, buildings, ships, etc. If the letter H follows the numbers, the alloy is hardened. An "L" in the center of the number means that the alloy contains lead (e.g. 12L14 is alloy 1214 with about 0.25 wt% lead). The addition of lead makes the alloy much easier to machine. Likewise a "B" in the center means that the alloy contains boron and a "BV" in the center means the alloy contains boron and vanadium. Nickel, chromium, and molybdenum are added usually in small amounts (<5 wt%) to enhance mechanical properties, fabrication characteristics, retention of strength at high temperature, or some other attribute. Their addition transforms the class of carbon steel into low alloy steel. In general, alloying elements that increase hardenability tend to decrease machinability. For example, addition of nickel and silicon (said to strengthen ferrite) can decrease machinability more than equivalent amounts of carbide forming elements such as chromium and molybdenum. Addition of sulfur can overcome some deficiencies in machinability. These alloying elements can have some effect on corrosion in some environments. Very often, one encounters a machinability rating for steels and other alloys tabulated as percent machinability. This rating was developed for turning operations a number of years ago. The reference steel was grade B1112/1212, a resulfurized and rephosphorized carbon steel in the as-rolled, cold drawn condition. It was given a rating of 100% for a 1 hour cutting tool life at 50 meters/min or 165 surface feet/min using a high-speed steel tool. All of the other alloys have a cutting speed below or above that speed to provide for a one hour tool life for the same cutting tool. These ratings probably cannot be duplicated today because of advances in steel-making and processing technology. They may not apply to higher speed machining, the type expected to be encountered with newer machines. Differences in heat treating would affect the rating. The index does provide a qualitative ranking. Carbon Content Carbon has the dominant effect on the machinability of carbon steels because this element determines the ability of the alloy to be heat treated and governs strength, hardness, and ductility (ultimate tensile strength, yield strength, and maximum elongation) that can be achieved. As a general rule, increasing carbon content increases its strength and the power consumption required for metal removal operations. The second two numbers in the specification (YY in XXYY) signify the nominal amount of carbon as 0.YY wt%. This meaning of the third and fourth digits pertains to all of the alloys specified by the AISI/SAE specification. Low carbon steels, those with 0.15 wt% or less carbon (SAE XXYY, YY≤15), are low in strength especially in the annealed condition and machine poorly because they are soft. They can become gummy and adhere to tools. Work hardening can improve machinability of these alloys. Steels in the 0.15 wt% to 0.30 wt% range (XXYY, 15≤YY≤30) are usually machined satisfactorily in the as-rolled, as-forged condition. The medium carbon grades (XXYY, 30≤YY≤55) tend to machine best if an annealing treatment is done to create the correct microstructure (mixture of pearlite and spherodite) that limits hardness. The high carbon grades (XXYY, YY≥55) require the proper microstructure (spherodized structure) for best machining. Hardened and tempered structures tend to make machining more difficult. The optimum conditions or microstructures for easiest machining of steels depends on the microstructure. (For example, there are reports of steel with 0.06 to 0.2 wt% carbon being best machined in the as-rolled condition and steel with a carbon level of 0.4 to 0.6 wt% being best machined in the coarse lamellar pearlite to coarse spherodite condition.) Note that all ranges above are nominal values. They can change depending on concentration of other constituents and the application or other fabrication requirements.Meaning of the Classification Numbers 1XXXThe first digit "1" when followed by a "0" (e.g. 10XX) signifies steel with no specially added alloying elements such as chromium or molybdenum. These alloys are considered plain carbon steel. The second two numbers signify the nominal amount of carbon as 0.XX wt %. For example, 1018 steel has a nominal carbon level of 0.18 wt% with a variation of several hundredths of a percent. The alloys denoted by 10XX tend to contain small, e.g. impurity, levels of manganese, phosphorus, and sulfur. These alloys tend to be easier to machine than their low alloy counterparts that have comparable carbon concentrations. The number 11XX means that the alloy is a resulfurized, free machining alloy which contains additional sulfur. The number 12XX signifies a resulfurized, rephosphorized alloy in which both the sulfur and phosphorus contents are increased. These alloys are used because of their good machinability. Higher sulfur levels can reduce the cutting forces and tool wear in many cutting applications. The sulfur combines with manganese to form manganese sulfide inclusions which enhance chip formation. Steels with higher phosphorus levels tend to drill more readily because of improved chip segmentation. The number 15XX refers to an alloy in which the upper bound on manganese is greater than 1.1 wt%. The actual amount of manganese could be less. The addition of manganese can increase the strength in the as-rolled state and the ductility in the heat-treated state depending on carbon level. These higher levels of manganese make hardening easier and sometimes make machining more difficult. 2XXX The first digit "2" signifies that the alloy contains additional amounts of nickel. If the second digit is a "3", the alloy contains about 3.5 wt% nickel. If the digit is a 5, it contains about 5 wt% nickel. Addition of nickel increases the tensile strength (highest strength achievable on a stress-strain curve) without a reduction in ductility (ability of the alloy to elongate under load). All of these alloys contain small amounts of manganese, phosphorus, sulfur, and silicon. 3XXX The first digit "3" signifies that the alloy contains additional amounts of nickel and chromium. Alloys designated as 31XX contain 1.25 wt% nickel and between 0.65 wt% and 0.80 wt% chromium. Alloys designated as 32XX contain 1.75 wt% nickel and about 1.0 wt% chromium. Alloys designated as 33XX contain 3.5 wt% nickel and 1.5 to 1.6 wt% chromium. Alloys designated as 34XX contain 3.00 wt% nickel and about 0.75 wt% chromium. The concentrations are nominal concentrations with a range allowed. All of the alloys contain small amounts of manganese, phosphorus, sulfur, and silicon. These alloys tend to be tough and ductile because of the nickel and exhibit increased wear and hardenability properties because of the chromium. Some corrosion resistance may be imparted by the chromium but the alloys are not like stainless steel as far as corrosion resistance. Chromium concentrations of higher than about 11 wt% to 12 wt% are required before the alloy behaves from a corrosion standpoint like stainless steel. 4XXX The first digit "4" signifies that the alloy contains molybdenum. The second digit signifies the other alloying elements. Following is a table denoting the expected alloying constituents and their approximate concentration range. All of the alloys contain small amounts of manganese, phosphorus, sulfur, and silicon. Molybdenum is a strong carbide former and has a large effect on hardenability and high temperature hardness. The time to form the correct microstructure tends to be increased resulting in an increase in quench time. The result is a deep hardening of the material during heat treatment. For example, the addition of about 1 wt% molybdenum tends to double the tensile strength. Depending on the concentrations, the alloys tend to possess good ductility, toughness, and wear resistance. This group of alloys is often encountered in machining operations. As a group, these alloys are machinable. Heat treating procedure and the resulting microstructure have the largest effect on machinability. For example, lean alloy steels (e.g. 40XX) in the as-rolled or annealed condition often machine like their carbon steel counterparts at similar carbon concentrations.
*Some of the constituent concentrations for alloys in this class vary from the amounts shown because of carbon concentration (0.XX wt%) or other considerations. 5XXX The first digit "5" signifies that the only additional alloying element is chromium. Alloys designated as 50XX contain 0.40 to 0.60 wt% chromium. Alloys designated as 51XX often contain either 0.70 to 0.90 wt% chromium or 0.80 to 1.15 wt% chromium. These concentrations can vary somewhat depending on the fabrication specification. All of the alloys contain small amounts of manganese, phosphorus, sulfur, and silicon. Several alloys are classified as 5XXXX. Alloy 50100 contains 0.40 to 0.60 wt% chromium, alloy 51100 contains 0.90 to 1.15 wt% chromium, and alloy 52100 contains 1.30 to 1.60 wt% chromium. These alloys contain about 1 wt% carbon and lower amounts of manganese (0.25 to 0.45 wt%). These alloys tend to be extremely hard and are more difficult to machine than those above. 6XXX The first digit "6" signifies that the additional alloying elements are chromium and vanadium. The only alloys that appear in this category are 61XX with nominally about 1.0 wt% chromium and often a minimum of about 0.15 wt% vanadium. The addition of vanadium improves the fatigue strength and elastic limit. At the low levels, there may be no effect on machinability. Heat treatment can affect machinability because of its effect on microstructure. For example, AISI 6150 machines best when in the annealed condition. 8XXX & 9XXX The first digit "8" or "9" (except for 92XX) signifies that the alloy is a triple-alloy steel containing molybdenum, chromium, and nickel. The following table shows the nominal ranges on the molybdenum, chromium, and nickel in these alloys.
Some variation in concentrations may occur in each class. Most of these alloys were developed during World War II. The alloys tend to exhibit qualities that are independently caused by each of the constituents. Manganese also varies from a range of 0.3 to 0.5 wt% up to 0.9 to 1.2 wt%. The alloy 92XX contains larger amounts of silicon, of the order of 1.80 to 2.20 wt%. This class may also contain small amounts of chromium in addition to the expected presence of phosphorus, sulfur, and manganese. At the levels in this alloy, silicon increases yield strength to make the steel better for structural applications. As stated previously, addition of silicon can make machining more difficult. Boron or Boron-Vanadium Alloys (XXBXX or XXBVXX) Alloys which contain boron or boron and vanadium are used for heavy sections or for materials used in mechanically severe applications. In these alloys, boron tends to be present at the 0.0005 wt% to 0.003 wt% levels. Vanadium is present anywhere from 0.10 wt% and higher depending on alloy. The small amounts of boron when coupled with carbon increase hardenability. This addition of boron and vanadium has not been reported to significantly affect machinability. Classification by UNS Number There are a number of classifications that exist for alloys. The Unified Numbering System was developed by ASTM and SAE to designate chemical composition. A UNS number has been assigned to each chemical composition of a metallic alloy. It is not a specification. The designation consists of one letter and five numerals. The letter signifies a broad class of alloys. The numerals define specific alloys within that class. For example, SAE 4140 is UNS G41400 and SAE 4140H is H41400. The reader is referred to "Properties and Selection: Iron, Steels, and High Performace Alloys", ASM Handbook, Volume 1 for more details on how to relate UNS numbers to the above classifications. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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David C. Silverman, Ph.D. - Primary Consultant |