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International steel casting specs being set.

International Steel Casting Specs Being Set

By 1992, the European Community will be a monolithic market of 320 million consumers. Euronorm Standards are being established, and will include criteria for steel castings.

The member states of the European Community (EC) have committed to an ambitious program to create by 1992 a single internal market of 320 million consumers. This program involves issuing almost 300 new regulations for business sectors ranging from machinery to services.

The forthcoming adoption of these regulations, or Euronorm Standards, as individual national standards is profoundly affecting the activities of all U.S. standardization groups, particularly ISO/TC 17 (International Standards Organization Technical Committee 17 - Steel). The CEN (European Committee for Standardization) Euronorm standards are being copied from such existing ISO standards as the chemical methods analysis developed by SC 1, the general delivery requirements from SC 20 and many detailed requirements from product standards developed by the other SCs. Although not mandatory, this is the practical way for the Europeans to proceed.

Because Europeans are borrowing from U.S. standardization groups, we in the U.S. are effectively influencing Europeans to be more compatible with American practices and capabilities. Thus, the ISO standardization process now makes it technically easier for U.S. producers and users of steel castings to compete with Europeans in international trade.

International Standards Organization

Before discussing ISO standardization procedures, a review of ISO is helpful.

The steel standardization activities of ISO fall under the scope of the work of ISO/TC 17/SC 11 - Steel Castings. The Secretariat of Subcommittee 11 is managed by the Steel Founders' Society of America (SFSA) for the American National Standards Institute (ANSI), the U.S. member representative for ISO. The reporting order is as follows:

* ISO Central Secretariat, Geneva, Switzerland;

* ISO/TC 17 Secretariat, JISI, Tokyo, Japan;

* ISO/TC 17/TC, ANSI (SFSA), Des Plaines, IL, U.S.A.

Definitions--The ISO defines Standardization as "an activity giving solutions for repetitive application to problems in the spheres of science, technology and economics, aimed at the achievement of the optimum degree of order in a given context." Generally, the activity consists of formulation, issuing and implementing standards.

The ISO constitution states that the object of the organization shall be to promote the development of standardization and related activities in the world to facilitate international exchange of goods and services and to develop cooperation in the sphere of intellectual, scientific, technological and economic activity.

ISO Activities--ISO has published about 7000 international standards. These standards are important for developing an international consensus on technological criteria and promoting international trade and technology transfer. International standards are beneficial only if they are used directly in trade or implemented in member countries' national standards. A recent ISO Council meeting adopted such a resolution, stating: "the Council urges the member bodies to take all possible steps to embody international standards in their national standards in order to promote coherent implementation on a world-wide basis."

The international consensus on technological criteria reached in ISO is highly valued by international bodies such as the General Agreement on Tariffs and Trade (GATT) and Customs Cooperation Council (CCC). GATT deals with international trade standards. ISO is considered to be one of the most appropriate international bodies to publish international standards.

CCC has developed a new nomenclature, and ratification for it is now pending. In its criteria for types of steels, there is a strong relationship between ISO/TC 17 standards and CCC nomenclature. This indicates how ISO's international consensus is widely valued in the world.

ISO and CEN Cooperation--The ISO Council decided recently to organize a meeting among the presidents and secretaries-general of ISO, IEC, CEN and CENELEC (European Electrotechnical Standardization Committee) to define a coordinated policy for the development of standardization activities at European and international levels.

The ISO/TC 17 executive committee had already taken up this matter at its 6th meeting in June 1987, and resolved to exchange information regarding the work of ISO/TC 17 and ECISS (European Committee for Iron and Steel Standardization) so that each organization might utilize its counterpart's works. The first round of the exchange has been successful, and it is believed that future information exchanges will contribute to the rapid development of international standards for both organizations.

ISO/TC 17-Steel--In terms of the number of standards published, the number of standards in process and the number of P-members, ISO/TC 17 - Steel is one of the largest and most active technical committees in ISO.

TC 17 was one of the ISO's first group of technical committees after it officially started in 1947. At its first plenary meeting in London in 1959, there was only one working group (WG) which dealt with testing methods, including tensile testing. It completed work begun by ISA 17, the International Federation of the National Standardization Associations - 17.

ISO/TC 17 has since published 68 standards and established numerous subcommittees. In 1975, one of these subcommittees was turned into a new technical committee, ISO/TC 164 - Mechanical Testing of Metals, because of the generic nature of the standards that the subcommittee handles.

The years from 1953-1958 were the beginning of attempts to standardize such internationally traded steels as structural steels (unalloyed steels) and heat treatable and alloy steels. The qualities, dimensions and tolerances were subjects to be specified. During the period, many working groups were set up for these steels.

Subcommittees to deal with the standardization of steels in product forms or steels used for specific applications were set up from 1961 to 1970 (SC 9 for tinplate and blackplate, through SC 15 for railway rails and their fasteners). As work progressed in each subcommittee, the need for delineation emerged. To discuss this and other matters of principle, WG 14, now called the Executive Committee, was formed.

Specific technical work, such as yield strength determination, basic rules for specifying impact strength and the International Numbering system for metals, was difficult for any existing SC to handle. Working groups to deal with those aspects were set up under the jurisdiction of the parent TC. Since 1974, there have been a series of moves within ISO/TC 17 to set up several subcommittees to deal with the standardization of steels used for other applications and product forms. The establishment of those subcommittees has enabled TC 17 to handle most all areas of steel standardization.

Steel Castings

Philosophy of Steel Casting Specifications--Steel foundries have an unique historical background that has led to the development of two philosophies of specifications. The foundry industry, as typified by iron foundries, existed by selling castings as finished or semi-finished products, with specifications generally assured by product performance.

The steel industry, as typified by large steel mills, has, on the other hand, existed by selling material in various forms with its specifications generally assured by material uniformity.

Therefore, the specification philosophy used depends on whether the steel foundry is identified as part of the foundry or the steel industry. In the U.S., steel foundries are generally considered part of the foundry industry. Specifications are based on mechanical properties, and meeting customer requirements is assured by product testing. Foundries have some freedom to trade off chemical composition and heat treatment to ensure that proper mechanical properties are achieved.

In most European countries, steel foundries are considered part of the steel industry. Specifications are written to ensure material uniformity so that customers know exactly which material is being supplied. Because these restrictions give a uniform product, final product testing is not always required.

There is considerable overlap in the requirements that these two philosophies develop. However, there remains conflict over which system delivers the appropriate quality and price to the customer. This conflict can be seen in international standards, where either set of requirements is allowed.

Types of Specifications--Regardless of the philosophy used to develop steel casting specifications, each has four basic types: material, test methods, acceptance criteria and delivery specifications. These requirements define a grade of material that can be ordered for a particular application. Test method specifications are used to ensure reproducible and accurate results and should not include any product requirements.

Test methods include chemical analysis, determination of mechanical properties, nondestructive inspection and similar tests. Chemical analysis and some mechanical properties requirements are usually contained in material specifications.

Other mechanical property determinations or nondestructive inspections usually allow the foundry and the customer to agree on acceptance values. Acceptance specifications give quality levels that may be used to set requirements, particularly for nondestructive inspection.

Specification Sources--A number of national and international specifications are used in Europe. They are the American Society for Testing Materials (ASTM), the British Standards Institute (BSI), the Deutsche Instut fur Normung (DIN) and the International Standards Organization (ISO). All four kinds of specifications are widely used, and their extensive acceptance shows that a working knowledge of all is useful (see Fig. 1).

Material Requirements

Mechanical properties and allowable chemical composition are normally found in the material requirements. Many similar materials are used, but there is no accepted worldwide method for naming ferrous materials and even less for designating cast ferrous materials. The two dominant nomenclature systems are the DIN and the AISI/SAE systems that were developed for wrought steels.

AISI/SAE designations are sometimes used to order cast alloy steels. It is possible to specify chemical composition by using ASTM A148 with an agreement as covered by paragraph 7.2, but care must be used to select a grade with compatible mechanical properties. Also, the wrought composition must be modified, especially the silicon and manganese content, to allow for casting.

This nomenclature system is useful since it allows the casting buyer to select a composition with which he is familiar and one that is compatible with properties of other components. In the U.S., most cast alloy steels are ordered to ASTM requirements. The structural grades in ASTM A27 and A148 are designated by their mechanical properties. The tensile strength/yield strength values designate the grade, such as 90-60.

As reflected in the grade designation, mechanical properties are the primary requirements. Alloy steels for pressure service is demanding, and other properties such as oxidation or creep resistance are important. The chemical composition and heat treatment are more restricted in this and similar specifications for pressure service. Alloy steels for elevated temperatures are ordered to ASTM A216 or A217.

Low temperature service requires strength and toughness to avoid sudden failures. Materials used at low temperatures must have controlled composition, heat treatment and should be tested before use. They are generally ordered to A352 or A757. Nickel is the most common alloy addition for toughness, along with restrictions on carbon, sulfur and phosphorus, and are usually quenched and tempered to give optimum toughness. Sometimes, stainless steel is required for low temperature service, typically to A351.

Stainless steels are commonly known by their AISI wrought names. These wrought numbers are used in BSI cast steel standards. In the U.S., cast stainless steels are named using the Alloy Casting Institute (ACI) system. An initial "C" or "H" shows whether the alloy is corrosion resistant (for service below 1200F) or heat resistant (for service above 1200F). A second letter indicates the approximate nickel-chromium location of either type on the iron-nickel-chromium ternary diagram shown in Fig. 2.

These letters advance from "A" to "Z" as nickel content increases. A numeral following the second letter shows the maximum carbon content of a C grade or the midpoint of a 0.10% carbon for an H grade. Additional letters after the numeral stand for special elements in the composition, such as M for molybdenum, C for columbium, Cu for copper and W for tungsten. Exceptions to this rule are "A" for controlled ferrite and "F" for free-machining.

The DIN uses the same nomenclature for all ferrous materials, cast or wrought. Carbon steels are designated by a C followed by the average carbon content (% x 100), e.g. AISI 1010 could be C10. In alloy steels, the average carbon content (% x 100) precedes the added alloy elements. The element symbols are then used, followed by the average content adjusted by a multiplying factor.

The factors are (% x 4) for Co, Cr, Mn, Ni, Si and W; (% x 10) for Al, Cu, Mo, Ti and V; and (% x 100) for C, N, P and S. For example, 8630 steel with 0.30%C, 0.50%Cr, 0.50%Ni and 0.20%Mo would have the DIN designation 30 NiCRMo 22. Steel with less than 0.1%Al, 0.25%Cr, 0.8%Mn, 0.50%Si and 0.1%Ti, is considered unalloyed. These factors apply only to low alloy steels, i.e., less than 5% alloy. For high alloy steels, an "X" is placed in front of the carbon content and no factor is used. An AISI 304 steel with 0.07%C, 18%Cr and 9%Ni would have a DIN designation of X 5 CrNi 18 8.

Cast steels are distinguished from wrought equivalents by a "G," for Guss, the German word for cast. Low alloy steels use the prefix "GS" for Guss Stahl, i.e., cast steel. For example, ASTM A216WCC would be GS C25. A cast 1.25Cr - 0.5Mo steel would be GS 17CrMo55.

Low alloy steels without specified chemical composition, but with required mechanical properties, are named by their average tensile strength. For example, GS-40 has a strength of 400 N/mm2. High alloy cast materials use a GX prefix. A typical high alloy steel, CF-8, or cast 304, with 18%Cr-10%Ni would have the designation of GX6CrNi1810.

Test Methods

The test methods used to determine a particular property can affect the value measured. Fortunately, there is general uniformity in mechanical testing procedures.

There are only two important variations in normal practice. The gage length of the tensile specimen can vary from standard to standard, and this will affect measurement of elongation. ISO and many other countries' standards have a longer gage length than ASTM, making ASTM measurements show relatively high elongation. It is important, therefore, to know which type of tensile specimen is required.

Another important variation is the use of a room temperature impact test as an indication of ductility, rather than reduction in area from the tensile test. Room temperature impact tests are required by some European codes. Therefore, the option of room temperature impact requirements or reduction of area requirements is contained in ISO documents. The casting purchaser must specify room temperature impact if such a code applies.

Acceptance Criteria

Even more of an issue than testing methods are the values of chemistry, physical properties, soundness and surface quality that will produce an acceptable casting. It is all too common to comply with all the customer's stated material requirements only to have the castings rejected because they did not meet expectations.

Whenever possible, specification-writing bodies should resolve such conflicts by establishing requirements that are meaningful to the customer, but still economically attainable by the foundry. The mechanical properties of a steel casting depend primarily on the interaction of casting design, section size, chemistry and heat treatment. Mechanical property requirements in materials specifications are arrived at by statistical analysis of test results from standard test bars.

It is commonly recognized that mechanical property values can decrease as the casting section size increases, especially toughness and ductility in carbon and low alloy steels.

In BSI standards, there is a note stating: "The mechanical properties required shall be obtained from test bars cast either separately from or attached to the casting to which they refer. The test values so exhibited represent, therefore, the quality of the steel from which the castings have been poured; they do not represent the properties of the castings themselves, which may be affected by solidification conditions and rate of cooling during heat treatment, which in turn are influenced by casting thickness, size and shape."

One source of conflict with customers is the difference in properties between the test bar and the castings. Some published guidelines are available, but more concrete specifications work on the properties and casting thickness relationship is needed. Either a table of requirements showing mechanical property minimums at various section sizes or a thickness limitation on already established minimums for various grades should be established.

The uniform thickness and shape of wrought steel forms allow tight specification of actual mechanical properties. Steel castings manufacturers should provide the design engineer with specification guidelines that reduce or eliminate uncertainty regarding actual casting properties in various sections. At present, the customer must have a special coupon made and tested if the sections are large and the customer wishes to account for this effect.

Conflicts may also occur when the customer checks the chemical analysis of the casting and finds it different from the reported heat analysis and outside specifications. Many times, the customer expects to analyze a chemistry very close to the heat analysis without realizing that the chemistry is not homogenous, and inaccurate test methods will cause differences that may be substantial. The customer may then resort to product check analysis tables like those found in ASTM A703 or A781 for low alloy steels.

A similar table for high alloy steels has been discussed in ASTM, and there are product check analysis tables in ISO DP4990. These tables do not present expected deviation of the measured casting chemistry from the heat analysis; rather, they present the permissible deviation of the casting from the specification limits.

The chemical and mechanical property requirements generally form the materials specifications. The mechanical integrity of a cast part is determined by nondestructive inspection techniques, such as visual, magnetic particle, liquid penetrant, radiography or ultrasonic evaluation. Visual inspection of a casting surface has always been a source of disagreement between manufacturer and purchaser because acceptance criteria are hard to define. Surface texture must be defined with a tactile indicator. Several indicators are available, like comparators from SCRATA (Great Britain), CTIF (France), CSC (U.S.) and CSC-C9 (U.S.). Surface discontinuities are more difficult to define. Pictures have been used in this country for MSS SP55 and ASTM A802.

The SCRATA comparators also are used as tactile standards to define surface discontinuities. ASTM A802 and MSS-SP55 have been revised to include the SCRATA tactile standards. Tactile comparators have proven to be less subjective than photographs, thus eliminating some disagreement between different inspectors.

There is no generally used acceptance criteria for liquid penetrant or magnetic particle inspections. Acceptance criteria have been proposed by the SFSA Specification Committee to ASTM, and are now being considered. The ASTM nuclear requirements are given as Level 1 in the proposal.

Some customer requirements specify no linear indications and this requires the removal and repair of all small indications. This not only adds cost, but the weld repair of small areas can cause hard spots or undetected underbead cracks which are more detrimental than the original condition. ISO/TC 17/SC 11 has defined acceptance criteria for magnetic particle (DP4986) and liquid penetrant (DP4987) examinations. The ISO requirements for Level 1 are very similar to the proposed ASTM Level 1.

Radiographic testing uses ASTM photographs to define acceptance levels. These are used throughout the world.

Ultrasonic testing has become an increasingly powerful tool for the evaluation of castings. Operator skill and the sophistication of the techniques used have made writing standards difficult, but ASTM and ISO have both done so.

ASTM A609 currently includes a method for longitudinal beam examination along with some suggested acceptance levels. This specification is being modified to increase the sophistication of the acceptance levels. ISO/TC 17 SC 11 also has developed levels for ultrasonic inspection in DP4992. This proposal is already used as the basis of the BSI standard.

Among the impediments to wider use of steel castings are problems encountered in machining, especially on transfer lines and numerically controlled systems. Some test methods and acceptance criteria must be established to identify unacceptable castings. Ultrasonic techniques might be useful in establishing such a standard.

There is no system that will automatically pick appropriate acceptance criteria for all casting applications. The service conditions and design must dictate the level of testing and values of acceptance. The designer and the customer should specify what they need, and then allow the manufacturer as much flexibility as possible in meeting those needs.

General Conditions

Almost every set of steel casting specifications carries a distinction between two classes of castings.

The "engineering-critical" or pressure-containing castings must have high levels of mechanical integrity and restricted chemistries, heat treatment and mechanical property requirements to ensure adequate performance. This level of quality is necessary since the failure of these castings can result in personal injury or large financial losses.

The "engineering-noncritical" nonpressure-containing or commercial steel castings are used in less critical roles, and while not less serviceable or lower in quality, require less verification and fewer restrictions. They are, therefore, less costly.

The definition of commercial casting quality is elusive, but a commercial castings should meet the applicable requirements in the general technical delivery document. In ASTM, nonpressure-containing castings generally must conform to A781 requirements. A commercial quality casting of an ASTM material conforms to A781 requirements. In ISO, DP4990 defines the general requirements for a commercial or general engineering casting. Pressure-containing or engineering-critical castings must conform to A703 in ASTM and to applicable code requirements. In ISO, pressure-containing castings must conform to DP4991.

There is some disagreement as to whether each heat must be tested to comply with all the requirements. In DP4990, several inspections are allowed. Nonspecific inspection leaves verification of meeting the material requirements to the manufacturer. Specific inspection requires the testing of each batch of castings, which may be done by heat. ASTM always requires testing of each heat to ensure compliance with all requirements.

These two systems reflect the key difference in responsibility and philosophy. If the specification restricts chemical composition to narrow limits, then mechanical testing could be less necessary. In this case, creep resistance and elevated temperature strength could be requirements of the specification without requiring specific testing of each heat. The manufacturer would have to test sufficiently to ensure compliance with all requirements.

In ASTM, composition and heat treatment are left more to the discretion of the manufacturer who, therefore, must test to show that each heat meets the required properties. The ISO steel standardization process continues to be attractive to the United States because it:

* enables U.S. producers to better understand international steel standards, and to influence what will be in ISO and Euronorm standards;

* gives U.S. users an opportunity to apply technically-acceptable standards not only from ASTM and SAE, but also from ISO;

* enables U.S. participants to gain insights that may benefit domestic standardization efforts. [Figure 1 to 2 Omitted]

John M. Svoboda, PhD. Process Metallurgy International, Inc Arlington Heights, IL
COPYRIGHT 1990 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
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Title Annotation:International Organization for Standardization
Author:Svoboda, John M.
Publication:Modern Casting
Date:Mar 1, 1990
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