A Comprehensive Guide to Nickel-based super alloy: Incoloy 800HT (UNS N08811/W.Nr. 1.4959)

What is Incoloy 800HT?

Incoloy 800HT, also known as “Alloy 800HT”, is designated as UNS N08811 or DIN W.Nr. 1.4959. It is an iron-nickel-chromium alloy having the same basic composition as Incoloy 800 and Incoloy 800H but with significantly higher creep-rupture strength. The higher strength results from deliberate control of the carbon, aluminum, and titanium contents in conjunction with a high-temperature anneal. Used in chemical and petroleum processing, in power plants for superheater and reheater tubing, in industrial furnaces, and for heat treating equipment.

Incoloy 800HT (N08811) is a kind of All-Austrian in the same series as Incoloy 800 a low-carbon nickel-iron-chromium alloy whose cobalt content can be strictly controlled below 0.01%. Incoloy 800HT (N08811) is resistant to corrosion by many corrosive media. Its high nickel content makes it have good stress corrosion cracking resistance under water-based corrosion conditions. The high chromium content makes it have better resistance to pitting corrosion and crevice corrosion cracking. The alloy has good corrosion resistance to nitric acid and organic acids, but has limited corrosion resistance in sulfuric acid and hydrochloric acid. 
In addition to the possibility of pitting corrosion in halides, it has good corrosion resistance in oxidizing and non-oxidizing salts. It also has good corrosion resistance in water, steam and mixtures of steam, air and carbon dioxide. Used in nitric acid condenser, resistant to nitric acid corrosion, steam heating, good mechanical properties, heating element tubes, good mechanical properties, etc. For applications up to 500℃, the alloy supply state is annealed state.

ASTM B408 UNS N08811/Incoloy 800HT rods. O.D=52 mm.

Incoloy 800, 800H & 800HT have nearly identical chemical composition and physical properties. The major differences between these nickel-based superalloys are their mechanical properties which stem from the restricted compositions of alloys 800H and 800HT and the high-temperature anneals used for these alloys. In general, alloy 800 has higher mechanical properties at room temperature and during short-time exposure to elevated temperatures, whereas alloys 800H and 800HT have superior creep and rupture strength during extended high-temperature exposure.

Incoloy 800HT serpentine coils for the heat exchanger.

Characteristics of nickel-based alloy incoloy 800ht (UNS N08811)

• (1) It has excellent corrosion resistance in extremely high temperature aqueous media up to 500°C.
• (2) Good resistance to stress corrosion.
• (3) Good processability.

Chemical Composition Requirements of Incoloy 800HT

The Chemical Composition of Incoloy 800, %
Nickel	30.0-35.0
Cromium	19.0-23.0
Iron	≥39.5
Carbon	0.06-0.10
Aluminum	0.15-0.60
Titanium	0.15-0.60
Manganese	≤1.50
Sulfur	≤0.015
Silicon	≤1.00
Copper	≤0.75
Al+Ti	0.30-1.20

Physical Properties of Incoloy 800HT

Density	Melting Range		Specific Heat		Electrical Resistivity
g/cm3	°C	°F	J/kg.k	Btu/lb.°F	µΩ·m
7.94	1357-1385	2475-2525	460	0.110	989

Temperature ℃	Specific heat capacity J/Kg.K	Thermal conductivity W/mK	Resistivity μΩ·cm	Elastic modulus KN/mm2	Coefficient of thermal expansion 10-6/K
20	440	10.8	112	195
93
100	462	12.4	114	190	14.1
200	488	14.1	118	185	14.9
204
300	514	15.6	120	179	15.2
316
400	540	16.9	124	174	15.6
427
500	565	18.3	126	168	15.8
538
600	590	19.6	126	161	16.0
649
700	615	21.0	127	154	16.7
760
800	655	23.2	128	142	17.2
871
900	680	25.7	129	130	17.6
982
1000	710	28.1	130	119	17.9

Mechanical Properties of Incoloy 800HT

Tensile Strength, min.		Yield Strength, min.		Elongation, min.	Hardness, min.
Mpa	ksi	Mpa	ksi	%	HB
600	87	295	43	44	138

The properties listed in the following table apply to the soft annealing (stabilizing annealing) of the specified specifications of Incoloy 800HT alloy. The special properties of non-standard size materials can be provided according to the requirements of specific applications. 

Mechanical properties at room temperature (minimum) (Test the minimum value of mechanical properties at 20℃)

Product	Specification	Yield strength  RP0.2 N/mm2	Yield strength RP1.0 N/mm2	Tensile strength  Rm N/mm2	Elongation A50 %	Brinell hardness  HB
Sheet and strip/ cold rolled  plate/ hot rolled	0.5-6.4	240	265	585	30	<=200
	5-100					135-165
Cold  processing and hot processing of bars	1.6-64
	25-100
	>100-240	220	250	550	35
Cold  processing and hot processing of pipes	64-240	180	–	530	30	–
	5-100	240	265	585	30
Condensation and  heat exchanger	16-76

High temperature mechanical properties (minimum)

Product	Yield strength RP0.2 N/mm2					Yield strength RP1.0 N/mm2
Temperature ℃	100	200	300	400	450	100	200	300	400	450
Plate, strip, tube	205	180	170	160	155	235	205	195	185	180
Baton	190	165	155	145	140	220	190	180	170	165

ISOV type notched impact test of Incoloy 800HT (UNS N08811)

Room temperature average value:

• Axial>=150J/cm²
• Radial>=100J/cm²

Time-temperature-sensitization curve:

Conditional stress value of Incoloy 800HT (UNS N08811)

The high-condition stress value that reaches 90% of the yield strength can be used in applications where a slightly larger amount of deformation is allowed. The permanent stresses caused by these stresses can cause dimensional changes, so it is not recommended for flange and gasket connections. 

Metallographic structure of Incoloy 800HT (UNS N08811)

Incoloy 800HT alloy has a stable face-centered cubic structure. The chemical composition and proper heat treatment ensure that the corrosion resistance is not impaired by sensitization. 

Corrosion resistance of Incoloy 800HT (N08811) alloy

Incoloy 800HT is a general engineering alloy, which is resistant to acid and alkali metal corrosion in both oxidation and reduction environments. The high nickel content makes the alloy have effective resistance to stress corrosion cracking. It has good corrosion resistance in various media, such as sulfuric acid, phosphoric acid, nitric acid and organic acids, and alkali metals such as sodium hydroxide, potassium hydroxide and hydrochloric acid solutions. The high comprehensive performance of Incoloy 800HT is manifested in nuclear combustion dissolvers with various corrosive media, such as sulfuric acid, nitric acid and sodium hydroxide, which are all processed in the same equipment.

Processing and Heat Treatment of Incoloy 800HT (UNS N08811)

Incoloy 800HT (N08811) alloy heating

• (1). Keep the workpiece clean before and during heat treatment.
• (2). Do not touch sulfur, phosphorus, lead and other low melting point metals during the heat treatment process, otherwise it will damage the performance of the material. Care should be taken to remove dirt such as marking paint, temperature indicating paint, colored crayons, lubricating oil, fuel, etc.
• (3). The lower the sulfur content in the fuel, the better. The sulfur content in natural gas should be less than 0.1%, and the sulfur content in heavy oil should be less than 0.5%.
• (4). Considering the need for temperature control and keeping clean, it is best to conduct heat treatment in a vacuum furnace or a gas shielded furnace.
• (5). It can also be heated in a box furnace or a gas furnace, but the furnace gas must be clean and neutral to slightly oxidizing. The furnace gas should be avoided from fluctuating between oxidizing and reducing properties. The heating flame cannot be directly heated. Burn to the workpiece.

Incoloy 800HT (N08811) alloy thermal processing

• (1). The thermal processing temperature of Incoloy 800HT ranges from 1200°C to 900°C, and the cooling method is water quenching or cooling as quickly as possible between 760°C and 540°C. Thermal bending should be carried out between 1150℃-1000℃.
• (2). In order to get the best corrosion resistance and creep resistance, annealing treatment should be carried out after hot working.
• (3). The material can be directly fed into the furnace that has been heated to 1200°C, and the holding time of the material is 60 minutes per 100mm thickness. After being kept for a sufficient time, the furnace will be quickly released, and thermal processing will be carried out in the specified temperature range. When the material temperature drops below the thermal processing temperature, it needs to be reheated.

Cold processing of Incoloy 800HT (N08811) alloy

• (1) The wor.k hardening rate of Incoloy 800HT is greater than that of austenitic stainless steel, so it is necessary to select processing equipment. The cold-worked material should be in an annealed heat-treated state, and intermediate annealing should be carried out during cold working.
• (2). If the amount of cold working is greater than 10%, the workpiece needs to be softened and annealed before use.

Heat treatment of Incoloy 800HT (N08811) alloy

• (1). The soft annealing temperature range of Incoloy 800HT is 920℃～980℃, and the best treatment temperature is 950℃.
• (2). In order to obtain the best corrosion resistance, the cooling method adopts water quenching, and the material with thickness less than 1.5mm can also adopt rapid air cooling.
• (3). During the heat treatment process, the operation must be performed in accordance with the aforementioned items that must be kept clean during the heating process.

Incoloy 800HT (N08811) alloy descaling and pickling

• (1). Incoloy 800HT has stronger adhesion of surface oxides and weld slag around the weld than stainless steel. Both mechanical and chemical methods can be used. Choose mechanical The method should avoid the method that will produce metal pollution or produce surface deformation.
• (2). Before pickling with HNO 3 /HF mixed acid, it must be carefully polished or salt bath pretreatment to break the oxide film.

Incoloy 800HT (N08811) alloy machining:

Incoloy 800HT must be machined after annealing and heat treatment. Due to the work hardening of the material, it is advisable to use a lower cutting speed and re-entry tool than processing low-alloy standard austenitic stainless steel in order to be machined under the cold work hardened surface.

Welding process of nickel-based alloy Incoloy 800HT (UNS N08811)

Incoloy 800HT (N08811) alloy is suitable for welding by any traditional welding process

Such as tungsten electrode inert gas arc welding, plasma arc welding, manual sub-arc welding, metal inert gas arc welding, melting electrode Inert gas shielded welding, shielded gas arc welding, etc. The welding of Incoloy 800HT must be carried out in an annealed state, and stains, dust and various marks must be cleaned up. With low heat input, the temperature between layers does not exceed 150°C. No pre- or post-weld heat treatment is required. 

Incoloy 800HT (N08811) alloy cleaning

remove oxide scale, oil stains and various marking marks, and use acetone to clean the base metal and filler alloys (such as welding rods) in the welding area. Note that trichloroethylene TRI and perchloroethylene PER cannot be used. And tetrachloride TETRA.  

Incoloy 800HT (N08811) alloy edge preparation

It is best to use machining, such as turning, milling, planing, or plasma cutting. If the latter is used, the cutting edge (welding surface) must be ground clean and flat, allowing overheating fine grinding. The base material on both sides of the weld should be polished to expose the bright metal. 

Incoloy 800HT (N08811) alloy groove angle:

Compared with carbon steel, the physical properties of nickel-based alloy and special stainless steel are mainly low thermal conductivity and high coefficient of expansion. These characteristics must be prepared during welding groove preparation. It should be considered, including widening the bottom gap (1-3mm), and due to the viscosity of molten metal, a larger groove angle (60-70°) should be used in butt welding to offset material shrinkage. 

Incoloy 800HT (N08811) alloy arc starting

The arc cannot be started on the surface of the workpiece, and the arc should be started on the welding surface to prevent corrosion at the starting point. 

Welding process of Incoloy 800HT (UNS N08811)

• (1) Incoloy 800HT is suitable for welding with the same material or other metals using any traditional welding process, such as tungsten electrode inert gas shielded welding, plasma arc welding, manual sub-arc welding, and extremely inert metal Gas arc welding, MIG welding, pulse arc welding is the first choice. When using manual arc welding, it is recommended to use (Ar+He+H₂ +CO₂ ) shielding gas mixed with multiple components. 
• (2) The welding of Incoloy 800HT must be carried out in the annealed state, and a stainless steel wire brush should be used to clean up stains, dust and various marks. When welding the root of the weld, in order to obtain the best quality of the root weld, the operation must be very careful (argon 99.99), so that the weld does not produce oxides after the root is welded. The color generated in the welding heat-affected zone should be brushed off with a stainless steel brush when the weld area is not cooled. Recommended welding materials: welding rod: EniCrFe-3, welding wire: ERNiCr-3. 
• (3) Incoloy 800HT welding parameters and influence (heat input): The welding operation should be carried out under the low heat input specified in the heat input table, using stacked bead welding technology, the temperature between layers does not exceed 120 ℃, and welding specifications must be followed. The heat input Q is calculated according to the following formula: 

    

• U = arc voltage, volt 
• I = welding current, ampere 
• V = welding speed, cm/min. 

Incoloy 800HT (N08811) alloy post-weld treatment (pickling, brushing and heat treatment)

The stainless steel wire brush should be used to remove oxides immediately after welding, that is, when the metal has not yet produced a welding color, the ideal surface quality can be obtained without pickling. If there are no special requirements or regulations, pickling is usually the last process in welding. Please refer to the section on descaling and pickling. No heat treatment is required before and after welding.

Product Forms and Standards of Incoloy 800HT

Product From	Standard
Rod and Bar	ASTM B408, EN 10095
Plate, Sheet & Strip	ASTM A240, A480, ASTM B409, B906
Seamless Pipe & Tube	ASTM B829, B407
Welded Pipe & Tube	ASTM B514, B515, B751, B775
Welded fittings	ASTM B366
Forgings	ASTM B564, DIN 17460

Application areas of nickel-based alloy incoloy 800ht (N08811)

• Heating pipes, containers, baskets and chains used in sulfuric acid pickling plants.
• Sea water cooling heat exchanger, marine product pipeline system, acid gas environment pipeline.
• Heat exchanger, evaporator, washing, dip tube, etc. in phosphoric acid production.
• Air heat exchanger in petroleum refining.
• Food engineering, chemical process.
• Flame retardant alloy for high pressure oxygen application.
• Heat exchange tubes
• Pipe fittings
• Flanges
• Valves

Varieties, specifications and supply status of Incoloy 800ht (N08811) nickel-based alloys

Variety classification:

Yaang Pipe Industry can produce Incoloy 800HT seamless pipes, Incoloy 800HT pipe fittings, Incoloy 800HT round bars, Incoloy 800HT forgings, Incoloy 800HT flanges of various specifications , Incoloy 800HT ring, Incoloy 800HT welded pipe, Incoloy 800HT steel strip, Incoloy 800HT wire and supporting welding materials.

Delivery status:

• Seamless pipe: solid solution + acid white, length can be set; plate: solid solution, pickling, trimming;
• Welded pipe: solid solution acid white + RT% flaw detection, forging: annealing + car polish; 
• Bars are forged and rolled, surface polished or car polished; strips are delivered after cold rolling, solid solution soft state, and deoxidized;
• Wire rods are finely ground in solid solution pickled disk or straight strips, solid solution straight strips Delivery in light state.

Incoloy 800HT iron-nickel-based alloy welding performance and welding process research

The welding characteristics of Incoloy 800HT were analyzed, and reasonable welding process and heat treatment process measures were formulated. Welding achieved good results after the transformation of styrene plant production stability and capacity to meet the design requirements of the load fully.

Shanghai SECCO Petrochemical Co., Ltd. needs to replace the static mixing element in the first reactor of 650kt/a styrene plant due to the static mixing element of the reactor is in a special position in the equipment; it is necessary to achieve the purpose of replacing the static mixing element by cutting 5 incisions, the material of the cutting line part is Incoloy 800HT. The static element is replaced at the site. Then, the cut lines are assembled and welded according to the sequence. Incoloy 800HT material in the Incoloy 800 and Incoloy 800H improved based on the ultra-high temperature with better creep resistance and durability. Due to the high alloy composition of 800HT, the welding process is prone to thermal cracking and reheat cracking, resulting in welding defects. Due to the role of the welding thermal cycle, carbide, and brittle phase precipitation is easy to occur, resulting in intergranular corrosion and joint embrittlement, affecting the organization and performance of welded joints. Construction through the bevel design, welding materials, welding process parameters, and heat treatment process parameters of choice successfully eliminated the 800HT post-weld thermal cracking and reheat cracking welding defects to avoid the generation of secondary hardening phase and to ensure the welding quality.

1. 800HT material properties and welding performance analysis

800HT mainly to increase the mass fraction of carbon (0.06% -0.10%) while controlling the mass fraction of aluminum and silicon and the sum of 0.85% -1.20%, and through appropriate heat treatment to obtain 5 or coarser grain size to achieve and maintain high creep resistance, durability and resistance to oxidation.
Due to the higher alloy composition, processing difficulties, and the purity of the smelting of material requires a high degree of purity, the S, P, Al, and Si content has more stringent restrictions on the chemical composition of Table 1. because S, P, Al, Si is easy to form a low-melting-point compounds with Ni, Fe and so on, resulting in weld cracks and crystalline cracks, the mechanical properties of the mechanical properties of the Table 2.
Table.1 Incoloy 800HT steel chemical composition

Element	Mass fraction
C	0.06-0.10
Si	≤1.0
Mn	≤1.5
Cu	≤0.75
S	≤0.015
Cr	19.0-23.0
Al+Ti	0.85-1.20
Ni	30.0-35.0

Table.2 Mechanical properties of Incoloy 800HT steel

Tensile strength (Rm)/MPa	Yield strength (Rd)/MPa	Elongation after fracture (A)/%
≥450	≥170	≥30

After welding thermal cycling, the heat-affected zone of iron-nickel-based alloy organization changes, carbide, and brittle phase precipitation, resulting in intergranular corrosion and joint embrittlement. Therefore, when welding such alloys, the weld zone should be avoided to stay in the high temperature for too long.
Thermal processing of the temperature range is very narrow; if the processing temperature is low, the deformation of residual stress will be released at high temperatures, accompanied by recrystallization under pressure will lead to instability while affecting the material’s high-temperature creep performance and durability of the material, so that the material performance degradation. If the temperature is too high, it will be accompanied by a secondary hardening phase (σ brittle phase and 475 ℃ brittle phase), thus making the material comprehensive mechanical properties, welding performance, and corrosion resistance decline.
800HT material welding requirements are relatively high due to its alloy composition being relatively highly prone to thermal cracking (crystallization cracking) and reheat cracking, so we should use a small amount of wire (less than 10kJ), no swing fast welding, multi-layer welding parameters, and should strictly control the temperature between layers. In addition, cobalt-containing electrodes should also ensure the weld metal’s high-temperature performance. At the same time, but also fully removes the welding bevel foreign matter. A strict drying electrode system controls the humidity of the welding environment, wind speed, weld height grinding, and so on.

2. Welding method and welding process selection

2.1 Welding method selection

Tungsten argon arc welding has a smaller welding heat input, shorter arc length, can be flushed behind the weld argon protection, and is almost suitable for any iron-nickel-based alloy welding. Welding electrode arc welding is mainly used for solid solution strengthened iron-nickel based alloy welding, electrode flux type for alkaline, due to the poor liquid metal mobility, to prevent the fusion and porosity and other defects, welding electrode appropriate swing.
After analysis, the transformation project Incoloy 800HT material welding using tungsten argon arc welding bottom ﹑ electrode filler cover method.

2.2 Welding material selection

In order to ensure that the performance of the parent material and the equivalent, welding materials and the chemical composition of the parent material should be as similar as possible, the choice of electrode ENiCrCoMo-1, the choice of wire ERNiCrCoMo-1, these welding consumables are Inco Inc. specifically for the Incoloy 800HT supporting the development to ensure that the weld with the parent material with the same high-temperature properties of the weld material selection is shown in Table 3.
Table.3 Welding material selection

Welding parts	Cut line
Steels	800HT
Welding rod grade	ENiCrCoMo-1
Specification/mm	∅3.2
Argon arc welding wire	ERNiCrCoMo-1
Specification/mm	∅2.5

2.3 Bevel form selection

Due to Incoloy 800HT for nickel-based materials, base material and its matching welding materials with higher nickel content, with lower thermal conductivity and high coefficient of expansion, the molten metal has viscous, the molten pool of metal mobility is worse, the chances of slag and porosity is greater, so the design of a larger bevel angle to offset the shrinkage of the material.
Cutting line material for Incoloy 800HT, according to the bevel selection chart (see Figure 1) for each part of the beveling process. Bevel processing using plasma and wheel-cutting processing should be carried out after the appearance of the inspection; the surface shall not have cracks, delamination, slag, or other defects.

Figure.1 Cutting line weld beveling

2.4 Welding process parameters

In the pre-welding of the material to fully baked, remove the water on the workpiece to reduce the presence of porosity; in the arrangement of the welding channel using a narrow weld channel, multi-channel welding, welding to minimize the joint constraints, limiting the swing of the welding rod to reduce the presence of slag, to prevent the production of thermal cracking, should be carried out in the provisions of the low heat input, the choice of a small current and line energy for welding (weld process parameters see table 4), control Interlayer temperature, arc extinguishing arc pit filling.
Table.4 Welding process parameters

Base material	800HT
welding method	GTAW	SMAW
Welding materials	ERNiCrCoMo-1	ENiCrCoMo-1
Specification/mm	∅2.4	∅3.2
Welding current/A	100-130	100-120
Arc voltage/V	12	22-26
Welding speed/(cm.min–1)	44993	45229

2.5 Mechanical properties of the joint test

Mechanical properties of welded parts test (tensile and bending test), Incoloy 800HT room temperature mechanical properties: Rm ≥ 450MPa, test data are in line with the standard requirements (mechanical properties test data in Table 5). To comprehensively assess the lateral deformation capacity of welded joints, according to the specification requirements for four side bending specimens, the results are shown in Table 6. In addition to a few specimens that appear to have permissible defects, most of the specimen’s tensile surface for the intact state, the results are qualified.
Table.5 Welded seam tensile test results

Sample number	L56	L56
Sample width/mm	25	24.6
Sample thickness/mm	24.7	24.3
Cross sectional area/mm2	617.5	597.8
Breaking load/kN	332.4	341.5
Tensile strength/MPa	540	570
Fracture location and characteristics	Welding seam	Welding seam

Table.6 Welded seam bending test results

Sample number	L56	L56	L56	L56
Sample type	Side bending	Side bending	Side bending	Side bending
Sample thickness/mm	10	10	10	10
Bending center diameter/mm	4a	4a	4a	4a
Bending angle/(°)	180	180	180	180
Test result	Qualified	Qualified	Qualified	Crack 2.5mm

High-temperature tensile test on the material, the data are shown in Table 7.
The conclusion is qualified by the provisions of the weld specimens test samples, determine the performance, and confirm the correct test records.
Table.7 High-temperature tensile test results of welded seams

Test number	L56-1	L56-2	L56-3	L56-4
Test temperature/℃	925	925	975	975
Rm/MPa	157	157	122	120
Rd/MPa	130	129	106	108
A/%	15	8	26	14.5
Reduction of Area (Z)/%	78	76.5	84.5	82

3. On-site welding and heat treatment process control

3.1 Welding process control

Tungsten argon arc welding bottom of the root layer of weld inspection should be carried out promptly at the next layer of weld welding to prevent cracks. Multi-layer multi-channel welding should be checked layer by layer after self-check qualification before welding the next layer until the welding is complete. Welding defects found should be removed promptly after rewelding. High chrome nickel alloy molten pool is viscous and difficult to flow; arc welding should take small swing measures. Both sides of the bevel should be a short stay to prevent the emergence of biting defects; each weld tries to finish the one-time welding. Interlayer temperature control at 40-80 ℃.

3.2 Appearance inspection and non-destructive testing of welded seams

The edge of the weld should be a smooth transition to the base material; the residual height shall not be greater than 2mm; the weld surface is not allowed to have a depth of more than 1mm sharp grooves and is not allowed to be lower than the surface of the base material. Weld surface shall not have cracks, pores, biting edges, fused, welded through, slag, or other defects; weld appearance check promptly for nondestructive testing; inspection method see table 8.
Table.8 Non-destructive testing

Position	Cut line
Material quality	Incoloy 800HT
Detection timing	Before pairing	After welding the bottom layer	After welding is completed	After heat treatment
Detection ratio/%	100	100	100	10
Test method	PT	PT	RT	PT、MT
Qualification level	NB/T 47013.5-Ⅰ	NB/T 47013.5-Ⅰ	NB/T 47013.2-Ⅱ	NB/T 47013.4/5-Ⅰ
Remarks			24 hours after welding

3.3 Heat treatment process control

Heat treatment parameters need to be strictly controlled. If the heat treatment is not appropriate, it will lead to the degradation of material properties. If the heat treatment temperature is low, or after the deformation of the material is not heat-treated, the material will not reach the grain size and stable organization, so the material’s high-temperature creep performance and durability of the decline in performance; and if the temperature is high, the ferrite content of the material will increase, and precipitation of σ-brittle phase and brittle phase at 475 ℃, so that the material performance degradation.
800HT material will show stress relaxation cracking sensitivity; welding processing will increase this sensitivity. When the design temperature is higher than 538 ℃ should be pressurized weld for a minimum of 885 ℃ post-weld heat treatment to remove stress relaxation cracking sensitivity.
After passing the test of the weld for post-weld stabilization heat treatment, heat treatment process parameters are as follows:

• Heating rate: 500 ℃ below, ≤ 150K/h; 500-850 ℃, ≤ 100K/h; 850 ℃ (30min), 0K/h; 850-910 ℃, ≤ 50K/h.
• Constant temperature: 899℃.
• Constant temperature time: 2h.

Heating using high-temperature-resistant special material resistance wire, the power arrangement of each pipe opening is 160kW, set up 4 partitions in the weld area (1 point each on the top and bottom, 2 points horizontally) for control, and set up an auxiliary heating area on each left and right side. The outside of the pipe is wrapped with aluminum silicate insulation material, and the thickness of the insulation is 150mm.

4. Conclusion

800HT material welding should be carefully planned and strict quality control under the guidance of the construction in a profound analysis of Incoloy 800HT weldability characteristics based on the development of a reasonable welding process and heat treatment process, successfully overcome the 800HT prone to thermal cracking and reheat cracking welding defects, and in the project to adjust the optimization, and ultimately obtained excellent welding quality. The qualified rate of non-destructive testing of welds and mechanical properties test indexes are better than the design indexes; after the styrene plant commissioning assessment, the modified reactor operates normally, and the output after commissioning fully meets the design load operation.
Author: Liu Qingshan