During the heat treatment process of steel pipes, various defects may occur due to improper process parameters, equipment failures, or operational errors. The following is a detailed analysis of common defects and their preventive measures:

I. Defects in the Heating Stage

1. Oxidation and Decarburization
   Defect manifestation: An oxide scale (Fe₃O₄, Fe₂O₃) forms on the surface of the steel pipe, and the loss of carbon elements leads to a decrease in surface hardness.
   Cause: Excessive air (oxidizing atmospheres such as O₂, H₂O, etc.) in the heating furnace, too high a heating temperature, or too long a holding time.
   Preventive measures:
   Use a controlled atmosphere furnace (such as N₂, Ar protective gas, or drip methanol and acetone to generate a protective atmosphere).
   Coat the surface of the workpiece with an anti-oxidation coating (such as a borax coating).
   Strictly control the heating temperature and time to avoid overheating.
2. Coarse Grain (Overheating)
   Defect manifestation: The austenite grains grow excessively, resulting in a decrease in mechanical properties (reduction in toughness and plasticity).
   Cause: The heating temperature exceeds the critical temperature by a large margin (for example, overheating occurs when the quenching heating temperature is 50~80℃ higher than the Ac3 line), or the holding time is too long.
   Preventive measures:
   Accurately set the heating temperature (according to the process specifications of the steel grade, for example, the quenching temperature of 45 steel is 840±10℃).
   Adopt stepwise heating or rapid heating technology to reduce the residence time at high temperatures.
   For overheated workpieces, they can be reprocessed through normalizing (to refine the grains).

II. Defects in the Cooling Stage

1. Quenching Cracks
   Defect manifestation: Transgranular cracks appear on the surface or inside of the workpiece, mostly occurring at the sudden change of the cross-section, sharp corners, or holes.
   Cause:
   The cooling rate is too fast (for example, when carbon steel is cooled below the Ms point, the martensitic transformation generates huge internal stress).
   The workpiece design is unreasonable (in stress concentration areas), it is not preheated, or there is machining stress before quenching.
   The cooling medium is not properly selected (for example, alloy steel is prone to cracking when quenched in brine).
   Preventive measures:
   Improve the structure of the workpiece (round off sharp corners, avoid large differences in thickness), and carry out stress-relieving annealing before quenching.
   Adopt isothermal quenching (bainitic transformation) or step quenching (first quench into a medium above the Ms point and hold it, then cool it slowly).
   Select an appropriate cooling medium (for example, 40Cr steel should be cooled in oil instead of water), and control the temperature of the quenching medium (for example, the oil temperature should not be lower than 40℃).
2. Insufficient or Uneven Hardness
   Defect manifestation: The surface or core hardness does not reach the standard, or the hardness difference in the same cross-section is >5HRC.
   Cause:
   Insufficient heating temperature (incomplete austenitization), insufficient holding time.
   Insufficient cooling rate (for example, the aging of the quenching medium and a large amount of impurities lead to a decrease in the cooling capacity).
   Surface decarburization (resulting in a lower hardness after quenching).
   Preventive measures:
   Strictly calibrate the temperature control instrument to ensure uniform heating temperature (the furnace temperature uniformity is ≤±5℃).
   Regularly replace or filter the quenching medium (for example, when the water content in the oil is >5%, the water needs to be removed).
   Control decarburization (the same as the oxidation prevention measures), and perform surface carburizing compensation if necessary.

III. Defects in the Tempering Stage

1. Tempering Brittleness
   Defect manifestation: The toughness decreases after low-temperature tempering (200~300℃) (the first type of tempering brittleness), or brittleness occurs during slow cooling after high-temperature tempering (450~650℃) (the second type of tempering brittleness).
   Cause:
   The first type: Carbides precipitate along the grain boundaries during the decomposition of martensite, which is irreversible.
   The second type: Impurity elements such as P and Sn segregate at the grain boundaries, which is reversible (reheating and rapid cooling can eliminate it).
   Preventive measures:
   Avoid tempering in the brittle temperature range, or use rapid tempering cooling (for the second type of brittleness).
   Select steel grades with low impurities, or add alloying elements such as Mo and W to inhibit grain boundary segregation.
2. Abnormal Hardness after Tempering
   Defect manifestation: The hardness after tempering is higher or lower than expected (for example, insufficient hardness after high-temperature tempering after quenching, or higher hardness after low-temperature tempering).
   Cause: Incorrect setting of the tempering temperature (for example, mistakenly setting 500℃ as 400℃), failure of the temperature control system, or insufficient holding time.
   Preventive measures:
   Calibrate the thermocouple and temperature control instrument before tempering (with an accuracy of ±1% FS), and use a dual-sensor monitoring system.
   Hold according to the process requirements (generally 1~2 hours, adjusted according to the cross-sectional thickness).

IV. Overall Defects: Deformation and Dimension Out-of-Tolerance
Defect manifestation: The steel pipe is bent, the ovality exceeds the standard (for example, the diameter tolerance is >±0.5%), and the straightness is >1mm/m.
Cause:
The self-weight of the workpiece or improper support during the heat treatment process (for example, a long steel pipe sags when heated while hanging).
There is a large temperature difference in the cross-section during cooling (for example, the cooling rates of the upper and lower surfaces are uneven during horizontal quenching).
Residual stress is not eliminated (for example, direct quenching of a cold-rolled pipe without annealing leads to stress superposition).
Preventive measures:
Use a vertical furnace to heat long steel pipes, or use supporting fixtures to keep them horizontal.
During quenching, rotate or swing the workpiece to ensure uniform cooling (for example, arrange annular nozzles during spray quenching).
Carry out annealing or normalizing before heat treatment to eliminate machining stress, and straighten the finished product (such as pressure straightening or tension straightening).

V. Preventive Management System

1. Process Standardization: Develop a “Steel Pipe Heat Treatment Process Card”, clearly defining parameters such as temperature, time, and medium (for example, the normalizing temperature of Q345B steel pipe is 880~920℃, and the holding time is 1min/mm).
2. Equipment Maintenance: Regularly calibrate the temperature control system (check once a week), clean the carbon deposits in the furnace (once a month), and check the stirring device of the quenching tank (ensure the flow rate ≥0.5m/s).
3. Process Monitoring: Use an infrared thermometer to monitor the surface temperature of the workpiece in real time, and randomly inspect the hardness (at least 3 points per pipe) and metallographic structure (for example, the martensite grade ≤3) for each batch.
4. Personnel Training: Operators must work with a certificate and master the heat treatment characteristics of different steel grades (for example, stainless steel solution treatment requires rapid water cooling to avoid the precipitation of the σ phase).

The core preventive idea for heat treatment defects of steel pipes is to precisely control the parameters of the three stages of heating, cooling, and tempering, and reduce stress concentration and organizational inhomogeneity. Through process optimization, equipment maintenance, and process monitoring, the defect rate can be effectively reduced, and the stability of the mechanical properties of steel pipes can be improved.

Galvanized seamless pipes are seamless steel pipes processed by hot-dip galvanizing or electro-galvanizing processes. They combine the high strength of seamless pipes with the corrosion resistance of the galvanized layer and are widely used in scenarios with high requirements for corrosion resistance and pressure resistance. The following is an analysis from two aspects: core characteristics and typical applications:

1. Core characteristics: anti-corrosion + high strength + easy processing

Excellent corrosion resistance

Hot-dip galvanized layer: coating thickness ≥85μm (national standard), forming a zinc-iron alloy layer, salt spray resistance test over 1000 hours, suitable for outdoor, humid or acid-base environments.

Electro-galvanized layer: coating is about 10-20μm, the surface is smoother, suitable for indoor light corrosion scenarios.

High strength and pressure resistance

The seamless process eliminates weld defects, the yield strength reaches 245-345MPa (depending on the material such as Q235, 20# steel), and the pressure resistance value can reach 10-30MPa, far exceeding ordinary welded pipes (pressure resistance ≤6MPa), suitable for high-pressure fluid transportation.

Smooth surface, low resistance

The galvanized layer is uniform and dense, and the inner wall roughness Ra≤1.6μm, which reduces the resistance of liquid flow and reduces energy consumption.

Long life and low maintenance

The outdoor life is up to 20-30 years (hot-dip galvanizing), saving more than 70% maintenance cost compared with ordinary steel pipes.

Easy to process

Can be cold-bent, welded, and drilled, suitable for complex structure processing.

2. Typical applications: Focus on high-demand scenarios

Construction projects

Scaffolding/steel formwork: Hot-dip galvanized seamless pipes are corrosion-resistant and rust-resistant, and are suitable for repeated disassembly and assembly in the open air (such as curtain wall keels for high-rise residential buildings: Hot-dip galvanized pipes are used in coastal buildings to resist sea breeze corrosion.

Underground drainage pipes: Buried pipes are resistant to soil corrosion and replace concrete pipes.

Machinery and equipment

Hydraulic cylinders: Seamless pipes have strong pressure resistance, and the galvanized layer is resistant to oil corrosion.

Transportation pipelines: The chemical industry transports acid and sewage, and the hot-dip galvanized layer is resistant to medium erosion.

Agricultural machinery: Sprayer brackets, greenhouse frames, and long-term rust prevention outdoors.

Energy and infrastructure

Oil and gas pipelines: Buried seamless galvanized pipes are resistant to soil electrochemical corrosion and are often used as oil and gas gathering and transportation branches.

Wind power/photovoltaic brackets: Galvanized seamless pipes are used inside the wind power tower at the seaside to resist salt spray corrosion.

Automobiles and transportation

Chassis structural parts: Hot-dip galvanized seamless pipes are lightweight and rust-resistant, and are used for SUVs Chassis longitudinal beam.

Highway guardrail: hot-dip galvanizing layer resists rust after collision, extending the replacement cycle.

Civil scene

High-end furniture: electro-galvanized seamless pipe has a smooth surface and is used for light luxury coffee tables and hangers.

Household water supply: replaces traditional PPR pipes, resistant to high pressure and no plasticizer precipitation.

III. Selection and precautions

Differentiation of process: hot-dip galvanizing is suitable for heavy corrosion protection, and electro-galvanizing is suitable for decorative scenes.

Welding treatment: The galvanized layer at the weld is damaged and zinc needs to be supplemented (such as zinc-rich paint) to avoid local corrosion.

Galvanized seamless pipes have “anti-corrosion + high strength” as their core advantages and are irreplaceable in high-requirement fields such as construction, energy, and machinery. When choosing, it is necessary to combine the environment (such as coastal/inland), pressure (such as low-pressure irrigation/high-pressure hydraulic), and cost for comprehensive decision-making.

Galvanized steel pipes used for domestic water supply are all connected by threads, also known as threaded connections. Generally, water supply pipes with a nominal diameter not exceeding 100 mm and a nominal pressure not exceeding 1 MPa can be connected by threads.

The threads of the connecting pipes are also divided into two types, cone and cylindrical, like pipe joints. If the general pipes are manually threaded, they usually do not meet the conical pipe threads.

The pitch and other technical requirements of cylindrical pipe threads are the same as those of conical pipe threads.

Pipe threads can be made by threading with pipe hinges or by lathes.

There are several different ways to connect pipe threads. For example, cylindrical internal threads are inserted into cylindrical external threads; cylindrical internal threads are inserted into conical external threads, and conical internal threads are inserted into conical external threads. The latter two methods can obtain a tighter connection, so they are also the most commonly used connection methods.

When pipes are connected by threads, appropriate fillers should be added between the external threads of the pipe and the internal threads of the pipe fittings. Commonly used fillers are oil hemp root and white thick paint. The specific method is: on the (external) thread of the sleeve, wrap a thin and even layer of oil hemp root along the direction of the thread, then apply white thick paint on the surface of the hemp root, and then screw on the connector. It must be noted that when using the hemp root white thick paint, it should be avoided to enter the pipe to avoid blockage.

After tightening the connector, in order to make the interface clean and beautiful, the excess oil hemp root and white paint should be removed and wiped clean. The use of oil hemp root and white thick paint not only plays a sealing role for the medium. It also plays a rust-proof role for the processed pipe thread.

Hot-dip pure zinc products, code is GI.

Las características de los productos de zinc puro por inmersión en caliente son una superficie bonita, una buena resistencia a la corrosión y una buena procesabilidad.

It is divided into two types, one is normal zinc spangles, and the other is zinc-free spangles. In the past, hot-dip galvanized products always had some zinc spangles on the surface because the lead in the zinc liquid could not be refined very pure, so our old concept is that hot-dip galvanizing has zinc spangles. With the needs of the automotive industry, if hot-dip galvanized automotive plates are to be painted, zinc spangles will have an impact on the painting. Later, by reducing the lead content in zinc ingots and zinc liquid to dozens of ppm, we can produce products with no or very little zinc spangles. For some special uses such as buildings, if you still like large zinc spangles, we can add elements such as lead or antimony to the zinc liquid to obtain large and beautiful zinc spangles.

Productos aleados, su código es GA.

The advantage of this product is that the paint adhesion of the coating surface is particularly good, the corrosion resistance after painting is also very good, and its weldability is also very good.

But its surface morphology is gray, which is not suitable for bare use. We do not recommend it if it is not painted, because its coating contains 7-15% iron. If it is not painted, this part of iron will generate a very light red rust. Although the red rust will not expand further in terms of corrosion resistance, the appearance is not very good.

Therefore, the main use of zinc-iron alloy is still in the use occasions with painting, just like the outer panel of the car and the side panel of the refrigerator. GA products can be directly used. For refrigerator processing, it can be directly powder-sprayed without pre-treatment, and the adhesion is also very good.

Aluminum-zinc products

The characteristics are particularly good corrosion resistance and beautiful surface appearance. Its zinc flowers look like beautiful fish scales, very beautiful, and can be used directly bare.

Its corrosion resistance is 2-6 times that of our ordinary hot-dip galvanizing. Its high temperature resistance is also relatively good. It can be used at 300℃ without discoloration. If used for a short time, it also has a better color retention ability at 700℃, and has excellent heat reflection effect.

Therefore, this kind of product is now widely used in the construction and home appliance industries.

Before hot-dip galvanized steel pipe welding, the hot-dip galvanized steel pipe is mainly heated, and welding begins after temperature control for 30 minutes. The heating, solid layer temperature and heat treatment process of welding are automatically controlled by the temperature control cabinet of the heat treatment process. The far-infrared crawler heat treatment furnace is used to automatically set the curve and describe the curve, and the thermal resistor measures the temperature.

During heating, the distance between the thermal resistance measurement points is 15mm-20mm from the weld boundary. In order to better prevent the hot-dip galvanized steel pipe from welding deformation, each column joint is welded symmetrically by two people, and the welding direction is from the middle to both sides.

When welding the inner opening, the first to the third layer must be operated in small specifications, because its welding is the primary cause of welding deformation.

After welding one to three layers, clean the reverse side. After removing the root with carbon arc air gouging, the weld needs to be mechanically polished and polished, and the weld surface needs to be combed for nitriding treatment to expose the metal texture to prevent the surface from being optimistically carbonized and cracked. The outer mouth needs to be welded once, and there will be some residual material in the inner mouth. When welding the second layer of hot-dip galvanized steel pipe, the welding direction should be opposite to that of the layer, and so on. Each layer of welding joints should be spaced 15-20mm apart.

When welding hot-dip galvanized steel pipes, the welding current, welding speed and number of welding stacks of the two welders should be consistent. When welding, the welding should start from the arc starting plate and end on the arc starting end plate. Cut off and grind and polish after welding.

Post-weld heat treatment: The heat treatment process should be carried out within 12 hours after the weld is welded. If the heat treatment process cannot be carried out immediately, insulation, slow cooling and other methods should be used. After the quenching and tempering treatment is completed, two thermal resistors should be used to measure the temperature, and the thermal resistors should be welded on both sides of the weld. According to the “Standards for Construction and Inspection of Steel Structure Engineering”, the welds are inspected by ultrasonic flaw detectors, and the inspection market share is 100%.