The Influence of Heat Input and T8/5 on the Notch Toughness and Strength of Welds
When welding heavy structural components or thick sections, real-world conditions are rarely as ideal as specified in the design. Although an approved WPS (Welding Procedure Specification) may prescribe a root opening of 4–5 mm, in practice, especially in shipbuilding or when working with distorted plates, root openings of 12 to even 16 mm are not uncommon.
A larger root opening inevitably increases the weld volume, requiring more energy input and resulting in a higher heat input. This directly affects the cooling rate (T₈/₅), a critical parameter that governs the resulting microstructure and, consequently, key mechanical properties such as notch toughness, yield strength, tensile strength, and hardness.
For welding engineers, a solid understanding of this parameter is essential to ensure consistent weld quality and reliable performance in demanding applications.
Heat input in welding is crucial because it directly affects the metallurgical changes and the final mechanical properties of the weld joint. The heat input is the amount of energy per millimeter of weld:
Heat-Input (kJ/mm) =
U = Voltage (V)
I = Current (A)
v = Travel Speed (mm/min)
A low heat input means rapid cooling, while a high heat input results in slow cooling. This slow or rapid cooling is related to the T8/5 cooling time.
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The T8/5 is the cooling time between 800 °C and 500 °C and determines the final microstructure and mechanical properties of the weld joint.
- A short T8/5 (rapid cooling) results in higher hardness in the HAZ (Heat Affected Zone), higher strength, lower notch toughness, and higher residual stresses.
- A long T8/5 (slow cooling) results in lower hardness, lower strength, higher notch toughness, and reduced residual stresses.
In practice, T8/5 varies from 5 to 30 seconds and depends on a number of important factors:
- Heat input
- Plate thickness
- Cooling (2D or 3D)
- The base material (thermal conductivity)
-
Preheat temperature
The higher the heat input, the longer the T8/5, and the slower the cooling.
The cooling rate T8/5 determines which phases form during solidification and cooling. During cooling between 800 and 500 °C, austenite transforms into other phases; if this happens too quickly, the steel does not have time to form soft structures such as ferrite and pearlite.
| T8/5 in sec | Typical structure | Properties |
| 2–5 | Martensite | Very hard and brittle |
| 5–10 | Bainite | Hard and reasonably tough |
| 10–25 | Fine ferrite + pearlite | Good toughness |
| 25 | Coarse ferrite | Low strength but high toughness |
For good notch toughness, one generally aims for an optimal, short T8/5 to limit grain growth and maximize the amount of grain boundaries. However, the T8/5 should not be too short, as this can lead to the formation of brittle martensite.
When a weld has to be executed with an excessively large root gap—for example, 12–16 mm instead of 5 mm—the welding process changes fundamentally: more filler metal must be deposited, and the travel speed decreases, causing the heat input to rise sharply (often exceeding 2.5 kJ/mm). As a result, the T8/5 will double or even triple, leading to a much slower cooling rate.
The consequence is a coarser microstructure, drastically reduced notch toughness, and a larger heat-affected zone (HAZ). Additionally, the HAZ remains above 723 °C for a longer period, which can cause undesirable grain growth and carbide precipitation.
Notch toughness indicates how much energy a material can absorb during fracture and is measured in Joules (J).
With a short T8/5 (rapid cooling), fine bainite or ferrite structures are formed that can absorb a large amount of energy. The most desirable structure that can form under these conditions is acicular ferrite (needle-like ferrite). This is a unique microstructure consisting of microscopically small needles that are completely interwoven in a criss-cross pattern. Think of it as a dense, impenetrable briar patch: when a crack tries to propagate through it, the crack tip constantly collides with needles oriented in different directions. The crack becomes hopelessly lost, is forced to constantly change direction, and eventually stalls. This gives the weld its legendary toughness, even in extreme freezing cold.
With a long T8/5 (>25 s), coarse ferrite grains form, which offer barely any resistance to crack propagation. A high number of small grains means many grain boundaries packed closely together. These grain boundaries force a crack to continuously change direction, which absorbs energy and slows down crack propagation, making the material tough.
Conversely, coarse grains offer very little resistance to crack propagation. Energy is poorly absorbed and the crack can propagate relatively easily, meaning the material is brittle.
| 0,8 kJ/mm | 7 s | 80–90 J | Very fine-grained, high toughness |
| 1,5 kJ/mm | 12 s | 60–70 J | Balanced structure |
| 2,3 kJ/mm | 20 s | 40–50 J | Clearly lower toughness |
| >2,5 kJ/mm | >25 s | <30 J | Coarse-grained, brittle behavior |
The result: with large root gaps (and consequently high heat input and T8/5, the notch toughness decreases, unless the welding filler metal has been specifically optimized for this.
A longer T8/5 not only leads to lower notch toughness, but due to the coarse grain structure (formation of coarse-grained ferrite and pearlite instead of fine, needle-like acicular ferrite), the yield strength (Re) and tensile strength (Rm) of the joint can also drop below the minimum required values.
With rapid cooling, a harder, stronger structure is formed (more martensite/bainite), but this comes at the expense of ductility and increases the risk of cold cracking.
Conversely, with slow cooling, the weld becomes less hard and more ductile at room temperature, but more brittle at low temperatures.A well-balanced weld metal maintains both strength and toughness within the desired range of:
- Tensile strength: 480–600 MPa
- Elongation: > 20%
- Notch toughness (-40 °C): ≥ 47 J
Properties of the HAZ: Why the Heat Affected Zone is critical
"The diagram below shows how the properties around a weld change: in the HAZ, peaks in hardness (susceptible to cracking) and drops in toughness (more brittle) occur, often making this the most critical zone of the weld."
In the CGHAZ (right next to the weld metal), you can see a massive peak in hardness (red line) and tensile strength (blue line). Because the material here cooled down very rapidly after extreme heating, a hard but very brittle structure (often martensite) is formed. This carries an increased risk of cold cracking.
Exactly at the point where the hardness is highest (in the coarse-grained CGHAZ), the toughness (green line) takes a sharp dive. Large grains in the metal are less capable of absorbing energy. This means that the material is locally very brittle here and can easily crack upon impact.
In the middle (the weld metal), the lines stabilize again at an average level. This is due to dilution: the weld metal is a homogeneous chemical mix of the base material and filler metal. The final properties here depend heavily on choosing the right filler metal.
The choice of welding filler metal largely determines how the weld behaves under high heat input and long T8/5 cooling times.CEWELD® AA R400 has been developed to perform well in these situations. This seamless rutile flux-cored wire has a balanced chemical composition that ensures:
- stable acicular (needle-like) ferrite formation, even with slow cooling;
- improved notch toughness down to -40 °C;
- low hydrogen potential (reduced risk of cold cracking);
-
a smooth, spatter-free weld appearance.
In a practical test, a 15 mm thick plate was welded with a root opening of no less than 15 mm and a heat input of 2.3 kJ/mm in the PF/3G position on a ceramic backing strip, using various brands of filler metals.The results for CEWELD AA R400 at -40 °C remained satisfactory, proving that the right chemical composition can compensate for the negative effects of a long T8/5 cooling time.
| Charpy V-Notch impact test [KV8] | |||||||
| Test method: ISO 148-1 | |||||||
| Specimen | Direction of specimen / Notch | Size (mm) | Temp | Absorbed Energy [J] | |||
| 48969-1 / 1,2,3 | Midwel ½ t | 10x10 | -40 | 1 | 2 | 3 | average |
| 38 | 51 | 52 | 47 | ||||
| 48969-2 / 1,2,3 Part A |
Midwel ½ t | 10x10 | -40 | 1 | 2 | 3 | average |
| 6 | 7 | 8 | 7 | ||||
| 48969-2 / 1,2,3 Part B |
Midwel ½ t | 10x10 | -40 | 1 | 2 | 3 | average |
| 9 | 9 | 7 | 8 | ||||
Kerfslagproef van verschillende merken lastoevoegmaterialen, Specimen 48969-1 is CEWELD AA R400
The T8/5 cooling time is the key to understanding the relationship between heat input and the mechanical performance of a weld. With large root openings, the heat input increases, cooling slows down, and the T8/5 increases, which can lead to coarser microstructures, lower notch toughness, and reduced strength.
However, this does not have to be a problem if:
- the welding parameters are carefully controlled
- the interpass temperature is limited
-
and the welding filler metal is tailored to these conditions.
The CEWELD AA R400 has proven that even with a T8/5 of >20 seconds and a heat input of 2.3 kJ/mm, excellent results can still be achieved, even at -40 °C.
"T8/5 is the thermometer of toughness — Control it, and the weld remains strong, even under extreme conditions"
In this blog post, we have seen how crucial controlling the heat input and the T8/5 cooling time is for a strong and tough weld joint. Would you like to accurately map out these parameters for your specific projects? Our advanced calculator helps you determine the ideal cooling time and preheat temperature.