For the purpose of this article, carbon steels are considered to be those steels in which carbon is the principal alloying element. Other elements that are present and that, in general, are required to be reported are manganese, silicon, phosphorous and sulfur. In a sense, all of these elements are residuals from the raw materials used in the manufacture of the steel, although the addition of manganese is often made during the steel making process to counter the deleterious effect of sulfur and silicon is added to aid in deoxidation.
The major source of sulfur is the coke used in the blast furnace or cupola. The major sources of phosphorous, manganese and silicon are the iron ore, the limestone used as
a flux, and the additions used to deoxidize the steel.
A minimum amount of silicon is needed to provide the necessary fluidity for the die casting operation. Other elements, particularly those not easily oxidized, e.g., copper and nickel, will be recovered from the scrap charge. The amounts of these two elements and others, such as chromium, molybdenum and vanadium, may or may not have to be reported by the manufacturer depending upon the specification.
Alloy steels are considered to be those steels to which elements, other than carbon, are added deliberately so as to improve mechanical properties, physical properties and/or corrosion resistance. (Mechanical properties are measured by plastically deforming or breaking the material. Physical properties are those, the measurement of which does not require that the metal be plastically deformed). The American Iron and Steel Institute has defined alloy steels as containing one, or more, of the following elements in quantities as follows:
|up to 3.99%
|up to 3.99%
and cobalt, columbium, molybdenum, nickel, titanium, tungsten, vanadium, zirconium, and any other element added to obtain a desired alloying effect.
The effectiveness of any alloying element is greatest when it is completely soluble in the steel. If a particular element forms, or tends to form, a compound with iron or another element present in the steel, the effectiveness of both elements is decreased. For example, if chromium is added to a carbon steel to increase hardenability, the austenitizing heat treatment must be at a temperature high enough to dissolve the chromium carbides, otherwise the presence of chromium carbides diminishes the effect of both chromium and carbon on increasing hardenability.
For all alloy systems, the mechanical properties are controlled by the chemical composition and the microstructure of the alloy. With respect to the carbon and alloy steels, the influence of microstructure is so great as to overshadow that of chemical composition and, for cast steels, the only practical method for changing the microstructure is by heat treatment. With few exceptions, the mechanical properties of the cast carbon and low alloy steels are controlled by heat treatment. Among the exceptions are the effect of carbon on increasing hardness, the effect of nickel on increasing toughness, and the effect of combinations of chromium, molybdenum, vanadium, and tungsten on increasing elevated temperature strength.
Hardenability-The major reason for using alloying elements in steels, wrought as well as cast, is to make the role of heat treatment on increasing strength effective over a wide range of metal thicknesses. This effectiveness is termed hardenability, i.e., a steel with low hardenability can only be strengthened by heat treatment in thin sections whereas a steel with high hardenability can be strengthened in thick sections.
Hardenability is the retarding of the austenite formation during cooling and is increased by all of the alloying elements, except cobalt. Hardenability, or austenite retention, should not be confused with austenite formation, which is the expansion of the austenite field at high temperature. Only a few elements increase the austenite field; these are carbon, cobalt, copper, manganese, nickel and nitrogen.
A large number of elements, particularly those that have a tendency to form carbides, delay the softening of steel that occurs during the tempering operation. The carbide forming tendency need not be strong. For example, silicon, not a particularly strong carbide former, will delay the softening during tempering, but its effect is not as strong as that of chromium; likewise, the effect of chromium is not as strong as that of vanadium, etc. In addition to retarding softening, some carbide forming elements will, during tempering, cause an increase in hardness due to the precipitation of the alloy carbides (precipitation hardening).
The role of the alloying elements on increasing hardenability and retarding softening during tempering was the subject of an extensive, international research effort during the first half of the 20th Century. Another result is the ability to estimate the hardenability of a given steel from calculations based upon the chemical composition of the steel. Multiplying factors for each element as published by the American Iron and Steel Institute are shown in Fig. 1.
Although these values were developed for wrought steels, they probably can be used for cast steels with some minor corrections. This figure does not illustrate the effect of carbon, which is quite large, nor the effect of boron, which is high for low carbon steels and is negative for high carbon steels (about 0.9% C) because it forms carbides.
In addition to alloying elements, the grain size of the austenite before quenching affects hardenability. A steel having a fine austenitic grain size has lower hardenability than a steel having a coarse austenitic grain size.
Although a steel having a coarse austenitic grain size has higher hardenability, a fine grain steel has better toughness, less distortion on heat treatment, lower tendency to develop quench cracks, and, probably, lower internal stresses. Of these characteristics, the improved toughness is the most important and, for structural applications, the steel should be of the fine-austenitic-grain type. A word of caution: The term aluminum-killed fine-grain steel often is misinterpreted. The term describes the grain size of the austenite at the heat treating temperature before quenching, not the grain size of the ferrite, tempered martensite, or martensite at room temperature. Methods for determining and measuring the austenitic grain size are described in ASTM Specification E112.
When heat treating aluminum-killed (aluminum deoxidized) steels, caution must be taken to avoid the use of austenitizing temperatures above about 1800F (982C), otherwise the aluminum nitrides will dissolve and a very coarse austenite grain size will result.
Hardness and Strength-All elements dissolved in steel increase its hardness and strength; however, as compared with the increase obtained by heat treatment, this effect is small. An exception is carbon, the effect of which is large, although not as large as the effect of heat treatment. Referring to the ASTM specification, A732/A732M, the effect of carbon on increasing the strength of annealed carbon steels is:
|Type of Steel
The carbon effect on the strength of quenched and tempered martensite in low alloy steel is more dramatic:
|Type of Steel
With respect to elements other than carbon, IC 4130 has about 1.2% total alloying elements; IC 4330 has about 3.0% total alloying elements; and IC 8630 has about 1.3 % total alloying elements, yet all have the same minimum tensile strength. This negligible effect of alloying elements on strength further emphasizes the previous statement that the most important role of the alloying elements in steel is their effect on hardenability.
Because of the close relationship between hardness and tensile strength of alloy steels, there could be a tendency to use carbon, an inexpensive element, to achieve a high strength steel. This tendency has some validity but entails some penalties. For example, at a given strength level, the toughness (resistance to brittle fracture) of a steel decreases with increasing carbon.
Consequently, a preferred plan is to select a steel having a combination of the lowest possible carbon content and the required amount of alloying elements to achieve, in a tempered martensite microstructure, the desired strength. Of course, the alloy content selected must be that required to achieve the hardenability needed for the section size (thickness) being considered.
Toughness-Toughness, the ability of a steel to resist brittle, rapid fracture, is an essential property for structural components subject to high loading rates. All of the test methods used to measure toughness involve the use of a sample containing a sharp notch or a crack. The reason for employing such samples is that for every service failure caused by brittle fracture, the source of the fracture has been found to be a discontinuity in the metal, and every engineering structure will contain some sort of one or more discontinuities (notches). Consequently, the test procedures used for toughness evaluations are aimed at determining the resistance of the metal to the rapid propagation of a discontinuity.
For structural carbon and low alloy cast steels, the following specifications either contain a toughness requirement or provide for a toughness test in the supplementary requirement:
For those specifications not mentioned, namely A216, A217, A389, and A487, the toughness (impact test) requirement of A703 may be used based upon agreement between the producer and the user.
Fatigue Strength (Endurance Limit)-For cast steels, the fatigue strength, or endurance limit, as determined by tests on smooth bars is generally in the range of 40-50% of the tensile strength. In non percentage terms, this relationship is expressed as 0.40 to 0.50 and is termed the endurance ratio. As illustrated by Table 1 the endurance ratio is largely independent of the tensile strength, chemical composition and heat treatment of the steel. Under conditions of rough surfaces, i.e., as cast, machined (not polished), notches, and cracks, both the endurance limit and the endurance ratio decreases. Fatigue strength values are affected by the mass effect in a manner similar to tensile strength. (See Alloy and Heat Treatment Influence on Section Size Effects.)
Section Size, Mass Effects-Mass effects are common to steels, whether rolled, forged or cast, because the cooling rate during the heat treating operation varies with section size, and because the microstructure components, grain size, and nonmetallic inclusions increase in size from surface to center.
The section size, or mass effect, is of particular importance to aluminum die castings because the mechanical properties are typically assessed from test bars machined from standardized coupons which have fixed dimensions and are cast separately from or attached to the castings. To remove test bars from the casting is impractical because removal of material for testing would destroy the usefulness of the component or require costly weld repairs to replace the material for testing purposes.
It cannot be routinely expected that test specimens removed from a aluminium casting will exhibit the same properties as test specimens machined from the standard test coupon designs for which minimum properties are established in specifications. The mass effect discussed above, i.e., the differences in cooling rate between that of test coupons and of the part being produced, is the fundamental reason for this situation. Several specifications provide for the mass effect by permitting the testing of coupons which are larger than the basic keel block.
Temper Embrittlement-When many alloy steels are heated in or slowly cooled through the range 800- 1100F (427- 593C), a significant loss in ductility and toughness occurs. This loss in toughness is believed to be caused by a grain boundary precipitation of iron compounds rich in phosphorous, arsenic, antimony, and tin; manganese and silicon enhance the action of these elements, and molybdenum retards the effect. Obviously, the use of melting practices to achieve low phosphorous (the practice to lower phosphorous also will lower arsenic, antimony and tin) is desirable, and such practices have been developed via ladle treatments. However, such practices are expensive, and for most applications the use of molybdenum and/or avoiding heating-in or slow cooling through the critical range 800- 1100F (475- 593C) is sufficient. When these practices are impractical, the use of the special melting-refining should be considered.
Hydrogen Effects-Hydrogen is an undesirable element in steel; none of the effects of hydrogen are good. Amongst the sources of hydrogen in steel, the most important are the raw materials used in the melting and refining process and traces of moisture in the molds used for casting. Care should be taken to dry all steelmaking and refining additions as well as molds.
When present in amounts as low as 3 parts per million, hydrogen in steel significantly reduces toughness and ductility. The hydrogen can be removed by heating the steel at about 400F (204C). However, the removal from heavy sections, e.g., above 8-in. (203.2mm), requires a very long heating time at 400F (204C), and because resistance to crack propagation decreases as the section size increases, hydrogen removal from heavy castings intended for severe structural applications is essential.
Fortunately, the attainment of the low amounts (less than 3 ppm) of hydrogen in steel can be achieved by the use of ladle vacuum treatments. Of course, the benefit of achieving a low hydrogen content must be sufficient to justify the high cost of the vacuum treatment.
Also, hydrogen can enter solid steel as a result of a corrosion reaction at the surface of the steel. When this occurs in steels having a hardness above 23 HRC, cracking may occur and the susceptibility of a steel to cracking increases rapidly as the hardness increases above 23 HRC. For this reason, the petroleum industry has imposed a maximum hardness restriction of Rc 23 on steels intended for service where corrosion will generate hydrogen absorption in the steel. One example is in sour crude wells.
Another form of hydrogen cracking of steel occurs when the steel is exposed to hydrogen at high pressure and high temperature. The steel does not need to have high values of hardness or strength. The physical mechanism at work, is that hydrogen, at the high pressure and temperature, diffuses into the steel and reacts with the carbides to form methane gases, the pressure of which becomes high enough to rupture the steel. The solution to the problem is to add alloying elements that form stable carbides which will not react with the hydrogen. The elements most commonly used are chromium, molybdenum, and, occasionally, vanadium.
Corrosion Resistance-The cast carbon and low alloy steels are not considered to be corrosion resistant materials. As is true for the wrought steels, minor additions of nickel, chromium, phosphorous, copper and silicon will increase resistance of cast steels to atmospheric corrosion to the extent that for some applications no protection such as paint is required. At elevated temperatures increasing amounts of chromium increase resistance to oxidation and, as already stated, low alloy steels for elevated temperature service contain chromium. Additions of aluminum and/or silicon are at least as effective as those of chromium; however, the amount of either of these elements required to impart the necessary oxidation resistance may cause a serious loss in ductility and toughness.
The role of the alloying elements in cast steel may be summarized as follows:
- All elements, except cobalt, increase hardenability.
- All carbide forming elements retard softening during tempering.
- Nickel improves toughness.
- Chromium improves oxidation resistance.
- Molybdenum retards temper embrittlement and increases elevated temperature strength.
- Vanadium and tungsten improve elevated temperature strength.
- Manganese plus sulfur improve machinability.
- Sulfur causes hot cracks.
- Carbon increases strength but decreases toughness.
- Phosphorous, arsenic, antimony, and tin accelerate temper embrittlement.
Structural carbon and low alloy cast steels enjoy a wide application for load bearing applications. The optimum utilization of these steels can be obtained best from a thorough understanding of the specifications covering these steels, and the metallurgical characteristics of the steel.
Both the manufacturer and the user have an obligation to cooperate in the preparation of specifications that reflect and combine both the needs of the users and the capabilities of the manufacturers. This obligation requires a sound understanding of the metallurgical characteristics of the steel, and a continuous communication between the two groups on the pertinent problems encountered in manufacture, fabrication and service performance.
* This article was excerpted with permission from Chapter 18 of the 6th Edition of the Aluminum Castings Handbook, published by the Steel Founders’ Society of America, and ASM International.