This presentation will provide the non-metallurgist with a basic understanding of carbon and low alloy steels. First we'll describe the carbon and low alloy steels by examining the iron-carbon binary phase diagram and understand the basic microstructures as related to carbon content. We'll discuss the nomenclature of the different carbon and alloy steel groups. We will then examine how mechanical properties are influenced through carbon content, alloy additions and heat treatment. We will also discuss the differences in carbon and low alloy steels that are specified as structural steels and high strength-low alloy (HSLA) steels. Finally, we will address the issues of material selection, processing and finishing.
1. Carbon and Low-Alloy Steels
Presented by Weldon ‘Mak’ Makela
Senior Failure Analysis Engineer
Materials Testing & Analysis Group, Element St. Paul
April 26, 2012 Carbon and Low-Alloy Steels
2. Future Topics for webinars
• Metallurgical Failure Analysis for Problem Solving-Dec. 4, 2011
• Carbon and Low-Alloy Steels-April 26, 2012
• Heat Treating
• Stainless Steels
• Tool Steels
• Aluminum Alloys
• Surface Engineering
• Corrosion
Carbon and Low-Alloy Steels 2
3. Carbon and low-alloy steels
• What is steel?
• Iron-carbon phase diagram.
• Carbon and low-alloy steel classifications.
• Mechanical properties.
• Microstructure.
• Application.
• Structural Steels.
• Specifications and selection of carbon and low-alloy steels.
• This presentation will not cover cast steels, coated products, forgings, cast
irons, ultra-high strength or other specialty steels.
• Tool steels and stainless steels will be covered in separate presentations.
Source: Metals Handbooks, 10th Edition, ASM International.
Carbon and Low-Alloy Steels 3
4. What is steel?
• Steel is iron with small amounts of carbon and other elements added to
impart unique properties in the material.
• Pure iron is soft, ductile and has low strength.
• Steel is made by reducing iron ore to iron, which contains carbon and
other impurities. Further refining reduces the impurities, controls
carbon and other element content.
• Steels consist of iron with varying amounts of carbon:
– Carbon content varies from 0.02-1.25%.
– Carbon is the primary elemental addition to increase strength.
– Carbon allows for heat treatment to increase strength.
• Other elemental additions improve properties:
– Manganese-up to 2.00%.
– Silicon-up to 1.0%.
– Chromium, nickel, molybdenum, and other elements in varying quantities.
Carbon and Low-Alloy Steels 4
6. Carbon Steels
• The most common metal used to manufacture products.
- Low-carbon steels: Carbon content varies from 0.05% to 0.30%.
- Medium-carbon steels: Carbon content varies from 0.30% to 0.60%.
- High-carbon steels: Carbon content varies from 0.60% to 0.95%.
• Other elements commonly found in carbon steels:
- Manganese is controlled to less than 2.0%.
- Sulfur is controlled to 0.35% maximum.
- Phosphorous is controlled to 0.12% maximum.
- Silicon is usually controlled to less than 0.60%.
- Lead, when added is controlled to less than 0.35%.
- Other elements are not controlled but are usually held to less than 2.0%.
Carbon and Low-Alloy Steels 6
7. Low-Alloy Steels
Elements are added to modify the basic carbon steel compositions to
provide superior properties.
• Manganese, silicon, chromium, nickel and molybdenum are the most
common additions to form low-alloy steels.
• Vanadium, niobium, aluminum, tungsten, copper and other elements
are added to provide additional specific characteristics.
• Total elemental additions are less than 10%.
Properties enhanced by alloying:
• Hardenability - the ability to be strengthened through heat treatment.
• Toughness - the ability to withstand impact loads.
• Environmental resistance - weathering and other corrosive
environments.
• Elevated temperature resistance.
Carbon and Low-Alloy Steels 7
8. Classifications of Carbon and Low-Alloy Steels
• Plain carbon Steels: Carbon, manganese, phosphorous and sulfur are
controlled. Other elements are not controlled.
• Resulfurized, rephosphorized or leaded steels: Sulfur, phosphorous or
lead are intentionally added to improve machineability.
• Low-alloy steels: Controlled additions of elements are utilized to
enhance properties and to provide specific characteristics.
• Structural steels: All steels could be used as structural steels but we
will focus on a group called the High-Strength Low-Alloy (HSLA)
Steels.
Carbon and Low-Alloy Steels 8
9. Classification of Steels
Classification can depend on:
• Composition―carbon, low-alloy, tool or stainless steels.
• Manufacturing method―open hearth, basic oxygen, electric
furnace, vacuum processed.
• Finishing method―hot or cold rolled, cold finished, cold drawn.
• Product form―bar, plate, sheet, strip, wire, tubing, or structural shape.
• Deoxidation practice―killed, semikilled, capped or rimmed.
• Microstructure―ferritic, pearlitic, or martensitic.
• Strength level―specified in ASTM or other standards.
• Heat treatment―annealed, normalized, spherodized or quenched and
tempered.
• Quality descriptors―commercial, forging, drawing, or aircraft quality.
Carbon and Low-Alloy Steels 9
10. Carbon Steel Nomenclature
SAE-AISI: Four digit designation.
• First 2 digits define the alloy group. For example:
– A 10 in the front indicates the group is a plain carbon steel.
– Resulfurized carbon steels start with 11, followed by the carbon content.
– Resulfurized and rephosphorized carbon steels will start with a 12, followed by the
carbon content.
– High manganese carbon steels will start with a 15, followed by the carbon
content for manganese contents between 1.00-1.65%.
• Last 2 digits indicate the nominal carbon content.
– Plain carbon steels will have the designation of: SAE 1005 – SAE 1095. This
indicates the nominal carbon content will vary from 0.05%-0.95%.
• AISI – American Iron and Steel Institute designation is slowly disappearing.
• SAE – Society of Automotive Engineers is more common.
• UNS – Unified Numbering System is a worldwide designation for composition
of metals and alloys. For example: UNS G10200 is the designation for SAE
1020 carbon steel.
Carbon and Low-Alloy Steels 10
23. General Comments on Impact Properties of Carbon
and Low-Alloy Steels
1. Carbon and low-alloy steels have a ductile-to-brittle transition
temperature:
- Above the DBTT the material will fail in a ductile manner and the
absorbed impact energy is high.
- Below the DBTT the material will fail in a brittle manner (cleavage)
with low absorbed energy.
2. The transition temperature can be shifted by alloy additions:
- Manganese and silicon will lower the DBTT.
- Sulfur and phosphorous will raise the DBTT.
3. The energy absorbed can be altered by alloy additions:
- Nickel will increase the toughness at low temperatures.
- Chromium, molybdenum and copper indirectly increase absorbed
energy through hardenability enhancement.
Carbon and Low-Alloy Steels 23
25. General Statements about Fatigue
Fatigue is a progressive, localized and permanent change in a material subjected to
fluctuating strains, at stresses with maximum values less than the ultimate tensile strength of
the material.
1. The stress can be substantially less than the ultimate tensile strength.
2. The alternating strains can lead to crack initiation and propagation.
3. As the crack grows in size, final failure can occur catastrophically when the remaining
cross section can no longer support the applied load.
4. Steels have a fatigue limit, which is approximately 50% of the ultimate tensile strength.
5. The following variables will affect the fatigue limit:
- Surface roughness
- Temperature
- Decarburization, carburizing, nitriding
- Designs that create stress risers
- Microstructure and grain size
- Material discontinuities
- Processing discontinuities
- Residual stress
- Corrosion or erosion
- Service-induced nicks or gouges
- Material properties, carbon content
Carbon and Low-Alloy Steels 25
34. Applications for Low-Carbon Steels
Low-carbon steels: Carbon content less than 0.30%.
• Products are sheet, strip, plate, wire, bar, tubing and structural shapes.
• Can be purchased in hot or cold-rolled, cold-finished, annealed, cold
drawn condition.
• Typical applications:
- Body panels for vehicles, appliances, etc.
- Coated products such as galvanized sheet, strip or wire.
- Low strength wire products.
- Structural shapes.
- Chain
• Weldable, formable, heat treatable to moderate strength levels.
Note: Low-carbon steels are often referred to as ‘mild’ steels.
Carbon and Low-Alloy Steels 34
35. Applications for Medium-Carbon Steels
Medium-carbon steels-carbon content between 0.30-0.60%.
• Increased carbon and manganese allow the medium-carbon steels to
be quenched and tempered to high strength levels.
• Purchased in many forms.
• Typical uses:
- Shafts, couplings, crankshafts, gears and other high-strength
applications.
- Rails, railway wheels, rail axles.
- Forgings, castings.
• Can be welded if properly pre-heated and post-heated.
Carbon and Low-Alloy Steels 35
36. Applications for High-Carbon Steels
High-carbon steels: Carbon content between 0.60-1.00%.
• High carbon allows heat treatment to very high strength levels.
• Cold working produces products with very high strength levels.
• Typical uses:
- Springs.
- High strength wire such as music wire.
- Tool applications-water hardening tool steels are commonly high -
carbon steels.
- Other products requiring high strength with a minimum of processing.
• Normally not weldable because of high-carbon content.
Carbon and Low-Alloy Steels 36
37. Applications for Low-Alloy Steels
Low-alloy steels: Carbon varies from 0.10-1.00%. Elements are added to
produce unique capabilities.
• Heat-treatable to high strength and toughness.
• Elemental additions can improve environmental degradation under
certain conditions.
• Elemental additions up to 10% can improve oxidation and corrosion
resistance at elevated temperatures.
• Common uses:
- Bearings and bearing races.
- Weathering steels.
- A myriad of parts and products that must be heat-treated to high-
strength or high-toughness.
Note: Low-alloy steels gain strength through heat treatment.
Carbon and Low-Alloy Steels 37
38. Structural Steels
High-strength carbon and low-alloy steels having yield strengths greater
than 275 MPa (40 ksi) and can be classified as follows:
• As-rolled carbon-manganese steels (13XX and 15XX).
• Heat-treated carbon steels.*
• Heat-treated low-alloy steels.*
• As-rolled high-strength low-alloy (HSLA) steels, also know as
microalloyed steels.
*Notice that we have been talking about carbon and low-alloy steels, but
now they are heat treated for use as high-strength structural steel.
Carbon and Low-Alloy Steels 38
39. High-Strength Low-Alloy Steels (HSLA)
Primarily utilized for structural applications requiring:
• High strength: HSLA steels utilize low carbon content with small amounts
of alloying elements and a variety of controlled processing parameters to
obtain high yield strengths, greater than 275 MPa (40 ksi.).
• Good toughness, weldability, formability and atmospheric and other
corrosion resistance.
• Availability as hot-rolled sheet, strip, and plate; hot-rolled and cold-finished
bar; tubing, pipe and structural shapes. Can also be furnished as cold-
rolled sheet and forgings.
Applications include construction of bridges, buildings, drilling rigs, vehicle
parts, piling, ships, etc.
Described in at least 18 ASTM specifications, which provide chemical
composition, mechanical properties, forms available and intended uses.
Many of these specs list several grades with different strength levels.
Carbon and Low-Alloy Steels 39
40. Specifications for HSLA Steels
ASTM Specification Available Forms Special Characteristics Intended Uses
A242 Plate, Bar, Shapes ≤ 4 in. Atmospheric weathering Welded, bolted or riveted construction
A572 Plate, Bar, Shapes ≤ 6 in. 6 grades with YS ≥ 42 ksi Bridges and buildings
A588 Plate, Bar, Shapes ≤ 8 in. Atmospheric weathering, YS ≥ 50 ksi Welded bridges and buildings
A606 HR & CR Sheet and Strip Atmospheric weathering Weight savings and durability
A607 HR & CR Sheet and Strip 6 grades with YS ≥ 45 ksi Weight savings and durability
A618 Welded and Seamless Tubing 3 grades with different characteristics Welded, bolted or riveted construction
A633 Plate, Bar, Shapes ≤ 6 in. 5 grades with YS ≥ 42 ksi Service down to -50°F
A656 Plate ≤ 5/8 in. YS ≥ 80 ksi Truck, crane, railroad car frames
A690 Piling Corrosion resistance Sea water exposure applications
A709, Gr 50 & 50W Plate, Shapes ≤ 4 in. Minimum YS = 50 ksi Bridges
A714 Pipe, welded and seamless 1/2 to 26 in. Pipe Piping
A715 HR Sheet, Strip 4 grades, YS = 50-80 ksi Structural, formability & weldability
A808 HR Plate ≤ 2 1/2 in. CVN 30-45 ft-lb @ -50°F Railway tank cars
A812 Coiled sheet YS = 65-85 ksi Welded pressure vessels
A841 Plate ≤ 4 in. YS = 45-50 ksi Welded pressure vessels
A847 Welded and Seamless Tubing YS ≥ 50 ksi Bridges and buildings
A860 Welded fittings YS ≥ 70 ksi Gas, oil transmission lines
A871 Plate ≤ 1 3/8 in. Atmospheric weathering Tubular structures and poles
Carbon and Low-Alloy Steels 40
41. Specifications for Carbon & Low-Alloy Steels
Specifications are written statements defining product requirements.
• Describes both technical and commercial requirements.
• Controls procurement.
• May cover any or all of the following parameters:
- Scope defines product classification, size range, processing, or other
information deemed useful to both supplier and user.
- Chemical composition of the carbon or low-alloy steel.
- Quality statement describes special requirements such as steel
quality, type and processing methods.
- Quantitative requirements identify chemical composition ranges,
mechanical and physical properties and test methods germane to the
application.
- Additional requirements may include such items as size and
straightness tolerances, surface and edge finish, packaging and
loading instructions.
Carbon and Low-Alloy Steels 41
42. Specifications, continued
Most existing specifications have been prepared by engineering
societies, associations, and institutions whose members
make, specify, purchase and/or use steel products. Some common ones are
listed below:
• Association of American Railroads – AAR
• American Bureau of Shipbuilding – ABS
• American Railway Engineering Association – AREA
• American Society of Mechanical Engineers – ASME
• American Petroleum Institute – API
• American Society for Testing and Materials – ASTM
• Society of Automotive Engineers – SAE
• Aerospace Material Specifications (of SAE) – AMS
• Federal and Military Specifications – FED and MIL
Foreign countries have their own material and process specification
systems, such as the DIN, JIS, BS, AFNOR, UNI, etc. Many of these
specifications reference some ASTM specifications.
Carbon and Low-Alloy Steels 42
43. Specifications, continued
ASTM is the most widely used specification system because they are
complete for procurement purposes. Most ASTM specs include
composition, mechanical, dimensional, quality and testing
requirements, or reference other ASTM specs that cover specific aspects
of a material.
• ASTM specifications are used worldwide.
• Some federal and military procurements are gradually transitioning to
ASTM specifications.
• Material descriptions use common SAE-AISI designations but also
include the UNS system to identify a material composition.
A common ASME specification is referred to as the Boiler and Pressure
Vessel Code. This code is the authority for any application involving the
design and construction of boilers, pressure vessels and associated
piping, including nuclear applications. Many ASME material specifications
closely parallel ASTM.
Carbon and Low-Alloy Steels 43
44. Carbon and Low-Alloy Steel Selection
Material and process selection should always be based on the following
considerations:
• Material strength with reference to operational loads, vibration,
temperature and environmental exposures.
• Processing parameters such as formability, weldability, machineability
and other fabrication considerations to produce the product.
• Form of material to most economically fabricate the product whether it
be sheet, strip, plate, bar, or structural shape.
• Availability of the material in the required form, quantity and price.
• Finishing processes such as painting, plating, heat treatment, etc.
Always use a material and/or process specification to procure or finish a
product.
Carbon and Low-Alloy Steels 44
45. Some General Comments
1. Resulfurized, rephosphorized or leaded steels are not generally
weldable or heat treatable.
2. The above materials should not be used in dynamic or cyclical
applications, especially in cold weather environments.
3. When designing products, ensure the maximum load is no greater
than 1/3 of the yield strength of the material and well below the fatigue
limit.
4. Never use a steel in the ‘as quenched’ condition. Always temper the
steel.
5. When welding, always use pre-heat and/or post-heating when the
carbon content is more than 0.30%.
6. A low-alloy steel is not significantly stronger than a plain carbon steel
with the same carbon content, in the same condition. Low-alloy steels
provide high-strength, only after heat treating. Save money if you
don’t need high-strength.
Carbon and Low-Alloy Steels 45
46. Contact us for further information
Weldon ‘Mak’ Makela Josh Schwantes
Senior Failure Analyst Metallurgical Engineering Manager
651 659 7275 651 659 7205
weldon.makela@element.com joshua.schwantes@element.com
Craig Stolpestad Mark Eggers
Sales Manager Inside Sales, NDT & Metals
651 659 7206 651 659 7349
craig.stolpestad@element.com mark.eggers@element.com
Carbon and Low-Alloy Steels 46