Aço inoxidável 304L vs 304H: uma comparação abrangente

304L vs 304H Stainless Steel: A Comprehensive Comparison

I. Introduction

During construction of a specific project in 2014, we encountered welding issues involving 304 and 304H pipe materials.

Through a series of project activities related to materials management, construction management, welding techniques, and inspections, we observed that 304 Series materials share common characteristics within a given range, but also have clear distinctions and strict separations.

Therefore, summarizing these similarities and differences contributes to the systematization of knowledge and the accumulation of future constructive experience.

304L vs 304H Stainless Steel: A Comprehensive Comparison

II. Introduction to 304 Austenitic Stainless Steel Materials

Austenitic stainless steel generally falls into the corrosion-resistant steel category and is the most commonly used type of steel.

Type 18-8 stainless steel is the most representative, exhibiting favorable mechanical properties and is convenient for mechanical processing, stamping and welding.

It offers excellent corrosion resistance in oxidative environments and good heat resistance. However, it is particularly sensitive to media containing chloride ions (Cl-), which can lead to stress corrosion cracking. 18-8 stainless steel, mainly 304, 304L, 304H, is available in various shapes, including sheets, bars and plates.

Alloy 304H (UNS S30409) is a modified version of austenitic alloy 304 with 18% chromium and 8% nickel. The carbon content in this product is controlled between 0.04 and 0.10, increasing the high temperature resistance of the product components in environments above 800°F.

This section mainly discusses the steel types, chemical composition and performance of 304 austenitic stainless steel materials, mainly comparing steel plates and steel pipes.

Approximate Equivalent Table of Steel Grades for Grade 304 Austenitic Stainless Steel Plates

No GB
24511-2009		 GB/T
4237-1992		 ASME(2007)
SA240		 PT
10028-7:2007
Unified numbering system New note Old series UNS Code Model Numeric Code Note
1 S30408 06Cr19Ni10 0Cr18Ni9 S30400 304 1.4301 X5CrNi18-10
two S30403 022Cr19Ni10 00Cr19Ni10 S30403 304L 1.4306 X2CrNi19-11
3 S30409 07Cr19Ni10 —— S30409 304H 1.4948 X6CrNi18-10

Approximate equivalent table of steel grades for class 304 welded austenitic stainless steel pipes

No GB/T
12771-2008		 GB/T
12771-2000		 ASME(2007)
SA312		 PT
10028-7:2007
Unified numbering system New note Old series UNS Code Model Numeric Code Note
1 S30408 06Cr19Ni10 0Cr18Ni9 S30400 TP304 1.4301 X5CrNi18-10
two S30403 022Cr19Ni10 00Cr19Ni10 S30403 TP304L 1.4306 X2CrNi19-11
3

Chemical Composition Table of 304 Austenitic Stainless Steel and Heat Resistant Steel

No Unified numbering system Chemical Composition (Mass Fraction) %
(Composition according to GB/T 20878-2007 standard)
W Yes Mn P s No Cr Mo N
1 S30408 0.08 1.00 2:00 0.045 0.030 8:00 ~ 11:00 6:00pm~8:00pm
two S30403 0.03 1.00 2:00 0.045 0.030 8:00~12:00 6:00pm~8:00pm
3 S30409 0.04~0.10 1.00 2:00 0.045 0.030 8:00 ~ 11:00 6:00pm~8:00pm
4 S30458 0.08 1.00 2:00 0.045 0.030 8:00 ~ 11:00 6:00pm~8:00pm 0.10~0.16

Table of allowable stresses for grade 304 austenitic stainless steel plate

Note Steel Sheet Standards Thickness
mm		 Allowable Stress/MPa at the Following Temperatures (°C)
≤20 100 150 200 250 300 350 400 450 500 525 550 575 600 625 650 675 700
S30408 GB24511 1.5~80 137 137 137 130 122 114 111 107 103 100 98 91 79 64 52 42 32 27
137 114 103 96 90 85 82 79 76 74 73 71 67 62 52 42 32 27
S30403 GB24511 1.5~80 120 120 118 110 103 98 94 91 88
120 98 87 81 76 72 69 67 65
S30409 GB24511 1.5~80 137 137 137 130 122 114 111 107 103 100 98 91 79 64 52 42 32 27
137 114 103 96 90 85 82 79 76 74 73 71 67 62 52 42 32 27
Note: The allowable stress in the first line is only applicable to components that allow slight permanent deformation.

Table of allowable stresses for grade 304 austenitic stainless steel tubes

Note Steel Sheet Standards Thickness
mm		 Allowable Stress/MPa at the Following Temperatures (°C)
≤20 100 150 200 250 300 350 400 450 500 525 550 575 600 625 650 675 700
S30408 GB13296 ≤14 137 137 137 130 122 114 111 107 103 100 98 91 79 64 52 42 32 27
137 114 103 96 90 85 82 79 76 74 73 71 67 62 52 42 32 27
S30403 GB13296 ≤14 117 117 117 110 103 98 94 91 88
117 97 87 81 76 73 69 67 65
S30408 GB/T14976 ≤28 137 137 137 130 122 114 111 107 103 100 98 91 79 64 52 42 32 27
137 114 103 96 90 85 82 79 76 74 73 71 67 62 52 42 32 27
S30403 GB/T14976 ≤28 117 117 117 110 103 98 94 91 88
117 97 87 81 76 73 69 67 65
S30408 GB/T12771 ≤28 116 116 116 111 104 97 94 91 88 85 83 77 67 54 44 36 27 23
116 97 88 82 77 72 70 67 65 63 62 60 57 53 44 36 27 23
S30403 GB/T12771 ≤28 99 99 99 94 88 83 80 77 75
99 82 74 69 65 62 59 57 55
S30408 GB/T24593 ≤4 116 116 116 111 104 97 94 91 88 85 83 77 67 54 44 36 27 23
116 97 88 82 77 72 70 67 65 63 62 60 57 53 44 36 27 23
S30403 GB/T24593 ≤4 99 99 99 94 88 83 80 77 75
99 82 74 69 65 62 59 57 55
Note: The allowable stress in the first line is only applicable to components that allow slight permanent deformation. The data corresponding to GB/T 12771 and GB/T 24593 were multiplied by the welding joint coefficient of 0.85.

When using austenitic stainless steel materials for pressure piping, it is essential to note that chromium-nickel austenitic stainless steel can undergo sigma phase embrittlement when used for a long period at temperatures between 540°C and 900°C.

It is advisable to control the ferrite content and the degree of cold deformation of austenitic steel. Under high temperature conditions (operating temperature greater than 540°C), low carbon austenitic stainless steel (C ≤ 0.08%) must also meet additional requirements:

1) The carbon content of the original material must be ≥ 0.04;

2) Heat treatment status: Rapid cooling >1040°C;

3) Average grain size should be grade 7 or coarser. If these additional requirements cannot be met, the allowable stress must be selected in accordance with the ultra-low carbon stainless steel.

Mechanical properties

High Temperature Yield Strength Table for Grade 304 Austenitic Stainless Steel Plate

Note Thickness
mm		 R p0.2 /MPa at the following temperatures (°C)
20 100 150 200 250 300 350 400 450 500 550
S30408 ≤80 205 171 155 144 135 127 123 119 114 111 106
S30403 ≤80 180 147 131 122 114 109 104 101 98
S30409 ≤80 205 171 155 144 135 127 123 119 114 111 106

High temperature yield strength table for grade 304 austenitic stainless steel pipes

Note R p0.2 /MPa at the following temperatures (°C)
20 100 150 200 250 300 350 400 450 500 550
S30408 210 174 156 144 135 127 123 119 114 111 106
S30403 180 147 131 122 114 109 104 101 98
S30409

In GB/T20878-2007 “Grades of stainless steel and heat-resistant steel and chemical composition”, a new high-temperature austenitic stainless steel grade S30409 (07Cr19Ni10) with a carbon content of 0.04% ~ 0.10% was added.

However, there is a dual classification problem with the 304 classification, that is, there is an overlap in carbon content between S30409 (07Cr19Ni10) and S30408 ​​(06Cr19Ni10). The same double classification problem also exists with S31609 (07Cr17Ni12Mo2), which has a carbon content of 0.04%~0.10%, and there is also an overlap in carbon content with S31608 (06Cr17Ni12Mo2).

III. 304 Austenitic Stainless Steel Material Management

Material labeling and quality assurance documents

Materials supplied and received for pressure piping must be labeled in accordance with corresponding standards and contractual stipulations. Each piece of material must have clear and firm markings, and for materials with a nominal diameter less than or equal to DN40, labeling may be done using tags or other substitute methods.

The content of the labeling must, at a minimum, include the manufacturer's markings and the material name (code). For austenitic stainless steel (type H) pipe components used under high temperature conditions, material lot numbers or codes must also be included.

Corresponding quality assurance documents include results of inspections and tests specified in standards and contracts and must be traceable.

4. Welding of 304 austenitic stainless steel

1. Welding Process Assessment

The base material for welding pressure support equipment is classified and grouped according to the chemical composition, mechanical properties and weldability of the metallic material.

According to NB/T47014-2011, the base material categories for S30403, S30408 ​​and S30409 are Fe-8, with a group of Fe-8-1. Welding process evaluation can refer to our existing welding process evaluation report HN2006-02-2012.

2. Selection of welding materials

The selection principle for welding materials is given in NB/T47015-2011: for welding high-alloy steel of the same type, welding materials must ensure that the mechanical properties of the weld metal are equal to or greater than the specified limit values to the parent material.

Where necessary, its corrosion resistance must not be lower than the corresponding requirements of the base material, or the mechanical properties and corrosion resistance must comply with the technical conditions stipulated in the design documents.

Recommended welding materials as shown in the following table:

Unified numbering system
(UNS)		 Note Shielded Metal Arc Welding (SMAW) Electrodes Submerged Arc Welding
(MOUNTAIN RANGE)		 Gas tungsten arc welding
(GTAW)
Electrode Models Example of electrode classes Flow Types Example of Flux and Welding Wire Classes Welding Wire Grades
S30408 06Cr19Ni10 E308-16
E308-15		 A102A107 F308-H08Cr21Ni10 SJ601-H08Cr21Ni10
HJ260-H08Cr21Ni10		 H08Cr21Ni10
S30403 022Cr19Ni10 ER308L-16 A002 F308L-H03Cr21Ni10 SJ601-H03Cr21Ni10
HJ260-H03Cr21Ni10		 H03Cr21N i10

3. Welding Considerations

Compared with carbon steel, the strength of austenitic stainless steel is 5 times that of carbon steel, resulting in more heat input under the same conditions of welding current and arc voltage.

Thermal conductivity is low, about 1/3 that of carbon steel, resulting in slow heat transfer and increased thermal deformation.

The coefficient of linear expansion is about 40% higher than that of carbon steel, which can easily lead to increased thermal expansion during heating and shrinkage during cooling, making the deformation after welding more pronounced.

Key points for welding austenitic stainless steel:

1) To avoid large welding deformations and stresses during welding, a welding method with concentrated welding energy should be chosen.

2) Strict control of welding heat input must be maintained to prevent the weld grain from growing severely and the occurrence of hot cracks in the welding.

3) To improve the thermal cracking and corrosion resistance of the weld, the welding area must be kept clean to prevent harmful elements from infiltrating the weld.

4) Austenitic stainless steel does not require preheating during welding. To prevent grain growth in the weld and heat-affected zone and carbide precipitation, and to ensure the plasticity, toughness and corrosion resistance of the welded joint, the inter-pass temperature must be kept low, generally not exceeding 100°C. .

4. Austenite grain problem

Austenite grain is the grain obtained when steel is austenitized, and the grain size is called austenite grain size. The standard grain size is divided into 8 levels, levels 1-4 are coarse grains, levels 5-8 are fine grains, and levels 10-13 above level 8 are ultrafine grains.

Observations are made under a 100x microscope. In actual production, grain refinement has become one of the important methods for strengthening metal materials, which can improve the strength and toughness of steel at the same time. We also hope to achieve finer grains during engineering welding.

We need to pay attention to the influence of heating rate during the welding process. The heating rate is essentially an overheating problem. The greater the degree of superheating, the greater the relationship between the nucleation rate and growth rate and the smaller the initial grain size.

Despite this, austenite grains tend to grow at high temperatures, so there cannot be a long residence time at high temperatures. Therefore, we emphasize rapid heating and cooling during welding.

The carbon content in the steel also affects the austenite grain. When the carbon content in steel is not enough to form undissolved carbides, as the carbon content increases, the austenite grains tend to grow and become coarser.

Therefore, among the three, S30408 ​​​​is most likely to thicken, and attention should be paid to the control and prevention of grain thickening in other aspects.

Controlling the chromium-nickel ratio in the weld metal, for 304 stainless steel, when the chromium-nickel ratio of the welding material is less than 1.61, hot cracking is likely to occur; when the chromium-nickel ratio reaches 2.3-3.2, hot cracking can be avoided.

Strictly limiting the content of harmful elements such as boron, sulfur, phosphorus and selenium in the weld metal can also prevent the occurrence of hot cracking.

V. conclusion

The design of petrochemical equipment for high temperature, high pressure and strong corrosion is demanding, and the choice of materials and welding construction must be carefully considered to ensure long-term stability of the equipment.

Understanding the characteristics of materials, distinguishing their differences and similarities and mastering specific welding construction techniques are particularly important.

With the development of materials technology, materials are increasingly optimized for specific performance, and “material selection and utilization” is becoming more specialized, as is knowledge. It is hoped that this article will play a positive role in this regard.

Conteúdo relacionado

Back to blog

Leave a comment

Please note, comments need to be approved before they are published.